ELEVATOR DOOR SYSTEM Technical Field [1] The present invention relates to an elevator door system, and, more particularly, to an elevator door system, which is equipped to an elevator entrance comprising a landing floor and a car, and which comprises a power-free coupling device equipped to the landing floor, a variable coupling device equipped to the car to push the power-free coupling device so as to open the elevator entrance when the car is horizontal to the landing floor, a speed reduction switch to allow the elevator entrance to be reduced in an opening speed and then completely opened when the elevator entrance is opened, and a pair of vertical clutch plates constituting the power-free coupling device and narrowed to allow the elevator entrance to be reduced in a closing speed and then naturally closed when the elevator entrance is closed, so that the elevator entrance is opened or closed while being reduced in speed at that moment. Background Art [2] Generally, a conventional elevator entrance comprises a jamb, a car door, and a landing door, in which conventional elevator includes an elevator car (hereinafter referred to as a "car") and a landing floor. When the car reaches the landing floor, the car door unlocks the landing door, and opens or closes the elevator entrance together with the landing door. [3] A conventional elevator door system comprising the car door and the landing door will be described with reference to the drawings. Hereinafter, the car door and the landing door will be described only at one side unless otherwise described. [4] Figs. 1 and 2 are rear views illustrating a car door 4 and a landing door 1 when the car door 4 and the landing door 1 are closed, respectively. [5] In Figs. 1 and 2, when the landing door 1 is in a closed state, it is locked by a locking lever 3 (unlocking lever) of a locking device 2. When the car door 4 is coupled with the landing door 1, a retiring cam 6 equipped to one end of a car frame 5 at one side of the car door 4 operates to unlock the landing door 1. [6] An unlocking roller 9 is provided to one end of a header case 7 of the landing door 1 corresponding to a location of a bending member of the retiring cam 6. The header case 7 is equipped with a shaft 9B. The unlocking roller 9 is attached to one end of the shaft 9B via a lever 9A. The unlocking lever 3 is equipped to either end of the shaft 9B. When the unlocking lever 3 is pushed by the bending member 8 of the retiring cam 6, the shaft 9B is rotated to allow the locking lever 3 to unlock the landing door 1. [7] Fig. 3 shows a locked state of the retiring cam 6 and the landing door 1 when the car is moved after the landing door 1 is closed, and Fig. 4 shows an unlocked state of the retiring cam 6 and the landing door 1 when the landing door 1 is opened. [8] Operation of a locking mechanism of the landing door 1 will be described as follows. When the car is moved after the landing door 1 is closed, the retiring cam 6 on the car is in a state wherein a rod 11 is lifted to retract the bending member 8 via excitation of a solenoid (an electromagnet) 10 in the retiring cam 6. [9] When the car reaches the landing floor, the solenoid 10 of the retiring cam 6 is released via a command from a controller (not shown), so that the rod 11 is pushed down by a spring 12 A, and the bending member is pushed forward by rotation of a link 12B. Then, as shown in Fig. 4, the unlocking roller 9 of the landing door 1 is pushed by the bending member 8, and the shaft 9B is rotated in the counterclockwise direction via the lever 9A, so that the locking lever 3 restricting movement of the car door 4 is rotated in the counterclockwise direction to unlock the landing door 1. When the landing door 1 is in an open state, the bending member 8 continues to push the unlocking roller 9. [10] When a command for closing the elevator entrance is sent from the controller (not shown), the solenoid 10 is excited to lift the rod 11, and then the bending member 8 is retracted as shown in Fig. 3. At this time, since the locking lever 9 is continuously subjected to a rotating force in the clockwise direction by a return spring (not shown), the locking lever 9 locks the landing door 1, as shown in Fig. 3. [11] As shown in Figs. 1 and 2, the landing door 1 is coupled with the car door 4 by a coupling plate 12 of the landing door 1 and a coupling device 13 of the car door 4. In other words, when a car door driving unit 14 equipped on the car frame 5 is driven, the lever 15 is rotated in the counterclockwise direction to force the rod 16 to rotate a link 17 about a pin 17a provided as a supporting point on the car frame 5, and then a cam integral to a small link 18A is rotated about the supporting point. As a result, (if the coupling device 13 is located at right in Fig. 1,) since a lever 19 of the coupling device 13 is rotated in the counterclockwise direction, the coupling roller 20 comes close to a stationary roller 21, so that the rollers 20 and 21 the landing door 1 and the car door 4 by positioning the coupling plate 12 therebetween. [12] For the conventional elevator door system constructed as described above, since a panel of the landing door and a panel of the car door are driven in the vicinity of their centers of gravity, respectively, it is possible to perform a stabilized door opening/ closing operation at high speed, and to enhance operating efficiency of the door system. However, the conventional door system has problems as described below. [13] Since the retiring cam is equipped to the end of the car frame, the unlocking roller must also be equipped to the end of the landing door, thereby increasing a width of a hoistway. [14] Since the retiring cam is equipped to the end of the car frame, the serviceman inevitably leans out above the car for installation or inspection of the retiring cam, possibly causing a danger of work. Similarly, when installing the unlocking roller of the landing door on every floor, it is also dangerous since the serviceman must lean out of the car or a scaffold for installation. [15] When the elevator entrance is in a closed state, the bending member must be lifted via excitation of the solenoid, increasing power consumption. [16] The header case of the landing door has a complicated construction, which increases manufacturing costs. [17] When unlocking the landing door, the bending member collides with the unlocking roller strike, thereby generating severe noise. [18] Since a motor of the car driving unit is protruded on the car, a work space for the serviceman is limited around the motor. As a result, there is a danger that a hand or a foot may be caught by the link lever when the link lever is rotated. Disclosure of Invention Technical Problem [19] The present invention has been made to solve the above problems, and it is an object of the present invention to provide an noise-free, easily installable and in¬ expensive elevator door system, in which a variable coupling device comprising clutch plates is equipped to one of car door holding plates holding left and right car doors with a fan belt interconnected with the car doors, and a coupling device for a power- free push roller is equipped to a landing floor corresponding to a car. [20] It is another object of the present invention to provide an elevator door system, which comprises a pair of clutch plates adapted such that, while the door is closed, one of the clutch plates is separated from the other and contacts a push roller, and at a moment that the door is completely closed, the clutch plates contact in parallel to each other, and forms a space to the push roller to reduce an opening or closing speed of the door, thereby preventing accidents from happening. [21] It is yet another object of the present invention to provide an elevator door system, which comprises a switch unit in conjunction with movement of a cam plate lever in¬ terconnected with the variable coupling device to command acceleration-reduction of the door, thereby allowing acceleration-reduction of the door with a simple con¬ struction. Technical Solution [22] For this purpose, an elevator door system is provided, comprising a car door having a variable coupling device, a landing door having a power-free coupling device inter¬ connected with the variable coupling device so as to open or close the doors with a simple construction, and a switch unit, wherein the power-free coupling device allows the landing door to be opened or closed via a rope subjected to force of a spring, the variable coupling device comprises a pair of clutch plates, one being fixed while the other is able to move against the fixed clutch plate, and allows the car door to be opened via a fan belt, and wherein the switch unit detects an operating state of the variable coupling device to accelerate or decelerate the door such that, when the car door is opened, the switch unit controls acceleration-reduction of the door and de¬ celerates the door at a final opening stage, and such that, when the car door is closed, a gap between the clutch plates is decreased to naturally decelerate the car door im¬ mediately before the car door is completely closed, thereby allowing safe operation of the elevator door system. [23] In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of an elevator door system, comprising a retiring cam equipped to one side of a car, a coupling device for opening/closing a car door in¬ terconnected with a motor at a center of the car, a separation coupling plate equipped to a landing floor while being interconnected with the coupling device, and an unlock roller interconnected with the retiring cam. More specifically, the elevator door system further comprises: a variable coupling device equipped to the car, and comprising a pair of vertical clutch plates, one being fixed while the other is able to move against the fixed clutch plate such that the clutch plates contact each other when the car door is closed and are separated from each other when the car door is opened, a clutch plate lever interconnected with the separable clutch plate to separate the separable clutch plate from the fixed clutch plate, if necessary, and a cam plate to drive a separating operation of the clutch plate lever; a power-free coupling device equipped to the landing floor to unlock the landing door by contacting and pushing a first push roller without power when the power-free coupling device is horizontal to the vertical clutch plates; and a switch unit equipped to a car frame to accelerate or decelerate the car door in conjunction with a switch shaft acting as a rotational shaft of a cam plate lever movably fastened at a lower end to one side of the cam plate when opening or closing the car door according to movement of the cam plate. Advantageous Effects [24] According to the present invention, the car door and the landing door are held by a wire rope and a fan belt in a closed loop, respectively, enabling the car door and landing door to be opened or closed with power of a single car door, and the car door is equipped with the pair of vertical clutch plates, one being fixed while the other is inter¬ connected with the cam plate and moved with respected to the fixed clutch plate, such that, when the car door is opened, an opening speed of the car door is reduced via detection of the switch unit, and such that, when the car door is closed, the separable clutch plate is separated from the fixed clutch plate at a middle closing stage, and then approaches the fixed clutch plate while decelerating the car door at a final closing stage, thereby reducing a final closing speed of the car door without an additional controller. [25] When the clutch plate pushes the push roller, hook coupling is released to open the car door, thereby allowing simple electric control of the door system. [26] The car door and the landing door are opened or closed by one door, thereby lowering manufacturing costs of the elevator door system. [27] An Opening/closing speed of the doors is controlled according to phases of ac¬ celeration-reduction cams having different phases by the switch unit in conjunction with rotation of the cam plate lever interconnected with the cam plate of the car door, in which each of the acceleration-reduction cams comprises an acceleration-reduction cam plate formed with elongated holes, and a tap plate formed with radial tap holes such that the tap holes and the elongated holes are coupled and adjusted by screws, thereby allowing easy adjustment of the phase angles. [28] The elevator door system of the invention further comprises an assistant stopper, which is formed with irregular cams contacting a stopper roller, thereby allowing smooth and safe closing of the car door. [29] The elevator door system of the invention further comprises a counterweight, which is provided on an upper end of the cam plate lever, and serves to reduce a load of the cam plate at the other end, thereby allowing initial opening or closing of the car door to be smoothly performed. Brief Description of the Drawings [30] The foregoing and other objects and features of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: [31] Fig. 1 is a rear view illustrating car doors of a conventional elevator door system; [32] Fig. 2 is a rear view illustrating landing doors of the conventional elevator door system; [33] Fig. 3 is a side view illustrating a closed state of the car doors and the landing doors; [34] Fig. 4 is a side view illustrating an open state of the car doors and the landing doors; [35] Fig. 5 is a rear view illustrating a power-free coupling device of a landing door of an elevator door system in accordance with the present invention; [36] Fig. 6 is a perspective view illustrating the power-free coupling device in accordance with the present invention; [37] Fig. 7 is a rear view illustrating a closed state of a variable coupling device of a car door of the elevator door system in accordance with the present invention; [38] Fig. 8 is a view illustrating a cam plate of the variable coupling device at a middle opening stage of the car door in accordance with the present invention; [39] Fig. 9 is a perspective view illustrating the variable coupling device of the present invention; [40] Fig. 10 is a view illustrating the cam plate of the variable coupling device of the present invention in a state of the car door being completely opened; [41] Fig. 11 is a perspective view illustrating a switch unit of the elevator door system in accordance of the present invention; and [42] Fig. 12 is a cross sectional view illustrating a coupling state of acceleration- reduction cams of the switch unit in accordance with the present invention. Best Mode for Carrying Out the Invention [43] Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. [44] Fig. 5 is a rear view illustrating a power-free coupling device of a landing door of an elevator door system according to the present invention, Fig. 6 is a perspective view illustrating the power-free coupling device according to the invention, and Fig. 7 is a rear view illustrating a closed state of a variable coupling device of a car door of the elevator door system according to invention. [45] In Fig. 5, a landing door 100 is equipped with a power-free coupling device IOOA shown in Fig. 6. The power-free coupling device IOOA comprises a first push roller 125, a hook bar 120 hingably fixed to a landing door holding plate 110 via a hinge pin 121 above the first push roller 125 movably fastened to the hook bar 120 while being hooked onto a hook coupling plate 130 fixed to a landing floor frame 103, and a second push roller 142 spaced a predetermined distance from the first push roller 125 and movably fastened to a fixed bar-fixing plate 140 fixed to the landing door holding plate 110. Here, the landing door holding plate 110 acts to hold the landing door 100 on an upper end of the landing door 100. [46] A car moves up and down in a hoistway, and has car doors, each of which is equipped with a variable coupling device 200A shown in Fig. 9. The variable coupling device 200A is coupled to the power-free coupling device IOOA. The hook bar 120 hingably fixed to the landing door holding plate 110 via the hinge pin 121 is provided at one end thereof with a spring 124 which generates a resilient force to allow the hook bar 120 to hook onto a hook hole 131, and is formed with a supporting groove 124 such that, when the hook bar 120 is disengaged from the hook hole 131, the holding groove 124 can be supported by a supporting bar 141. A roller 111 acts to guide a return rope 112. The return rope 112 is connected at an upper end to the hook coupling plate 130, and at a lower end to a tension spring 113 equipped at the bottom of the landing door 100 to pull the return rope 112. A wire rope 105 has a closed loop shape, and acts to hold the landing door holding plate 110 and to supply power to open or close the landing door 100. The wire rope 105 is guided along a roller 106. A roller 114 is equipped to the landing door holding plate 110, and guided along a horizontal rail 107. [47] In Fig. 7, the variable coupling device 200A comprises a pair of vertical clutch plates 210 and 211 facing each other between the first and second push rollers 125 and 142 and being in conjunction with opening/closing of the landing door 100 such that the clutch plate 210 is fixed and the clutch plate 211 is separable with respect to the clutch plate 210, a clutch plate lever 220 movably fastened at the center thereof to a clutch plate lever supporting plate 230 coupled to the car door holding plate 201 and having the clutch plate 211 movably fastened to a lower end of the clutch plate lever 220 such that the clutch plate 211 is separated from the clutch plate 210 in a gap between the first and second push rollers 125 and 142, a cam roller 240 coupled to a cam roller holder 241 coupled to an upper end of the clutch plate lever 220, and a cam plate 250 movably fastened to the car door holding plate 201 via a cam hinge pin 253 while being movably fastened to a lower end of the cam plate lever 260 via a lever hinge pin 261. The cam plate 250 is formed with a first cam groove 251 and a second cam groove 252 such that the cam roller 240 engages with the first and second cam grooves 251 and 252 while moving thereon. A lower end of the cam plate lever 260 is movably fastened to a switch shaft 262 of the switch unit 275 fixed to the car frame 270. [48] The clutch plate lever 220, which is movably fastened to the clutch plate lever supporting plate 230 fixed to the car door holding plate 201 such that the clutch plate 211 is separated from the clutch plate 210, is equipped at a lower end of the clutch plate lever 220 with a tension spring 255 which allows the cam roller 240 to move along the first and second cam grooves 251 and 252 while contacting the cam plate 250 with respect to a clutch plate lever hinge pin 214 located at the center of the clutch plate lever 220. [49] In Fig. 7, a driving unit of the elevator door system is provided on the top of the car frame 270, and comprises a motor 271 to supply power to open or close the car door, reduction pulleys 273 and 274 to supply power from the motor 271 to a pulley belt 272, a fan belt 281 coupled in a closed loop between the reduction pulley 274 equipped to one end of an upper frame 280 of the car door 200 and a roller 282 equipped to the other end of the upper frame 280, and fan belt holders 283 and 284 provided to each car door holding plate 201 such that each of the fan belt holders 283 and 284 is fixed to the fan belt 281 so as to interlock with the fan belt 281. The car door holding plate 201 comprises a roller 207 guiding a horizontal movement, and a rail 285 horizontally equipped to the upper frame 280 to allow movement of the roller 207. [50] An assistant stopper 290 is additionally equipped to a car door holding plate 202 coupled to the upper end of the car door 200, and has at least one stopper cam formed thereon. A roller holding plate 294 is slantly coupled to the upper frame 280 facing the assistant stopper 290 by a spring 297, and has a stopper roller 293 movably fastened to an end of the roller holding plate 294 such that the stopper roller 293 can be reduced in speed and then stopped via surface contact with the assistant stopper 290. [51] A counterweight 262 is equipped to an upper end of the cam plate lever 260 (opposite to the lever hinge pin 261 about the switch shaft 262 of the cam plate lever 260), and acts to reduce an initial operating load. [52] Fig. 11 is a perspective view illustrating a switch unit of the elevator door system, and Fig. 12 is a cross sectional view illustrating a coupling state of acceleration- reduction cams of the switch unit of the invention. [53] The switch unit 275 comprises acceleration-reduction cams C equipped on the switch shaft 262, and limit switches 276 located corresponding to the acceleration- reduction cams C, respectively. Each of the acceleration-reduction cams C comprises a tap plate 277 which has tap holes 277-1 radially formed in an arcuate shape and is inserted by the switch shaft 262, a fixing plate 279 adjacent to the tap plate 277 and having a relatively smaller diameter, a cam plate 278 fitted to an outer periphery of the fixing plate 279 and adapted to control a phase of a cam section 278-5 with the tap holes 277-1 and control screws 278-2, and a fixing bolt 263 fixed to the switch shaft 262 to fix the tap plate 277 and the cam plate 278. [54] Since the fixing plate 279 is adjacent the tap plate 277 and enables rotation of the cam plate 278 while holding the fixing bolt 263, it may be integrally formed with the tap plate 277, if necessary. An assistant clutch plate lever 220-1 is provided above the clutch plate lever 220 to assist separation of the vertical clutch plate 211, and is hingably coupled to the clutch plate 211 movably fastened by the clutch plate hinge pin 212 such that the clutch plate 211 is separated from the clutch plate 210. [55] With the elevator door system of the invention having a construction as described above, as shown in Figs. 5, 6, and 7., when the car starts to move vertically in the hoistway after a passenger opens the elevator entrance, and gets on the car, the clutch plates 210 and 211 of the car door shown in Fig. 7 are located between the first and second push roller 125 and 142 of the landing door shown in Figs. 5 and 6. In this state, when pushing an open button in the car, or when the motor 271 is operated by a command for opening the elevator entrance from the outside, the fan belt 281 is moved in the direction indicated by the arrow to open the landing door holding plate 201. When the vertical clutch plate 211 is in motion, the cam plate 250 is moved from a position of the first cam groove 251 to a position of the second cam groove 252 and the cam plate lever 260 is rotated in the counterclockwise direction as shown in Figs. 8 and 9, so that the vertical clutch plate 211 is separated a predetermined distance from the fixed vertical clutch plate 210 as shown in Figs. 8 and 9. [56] Before the above state, the first push roller 125 of Figs. 5 and 6 is pushed rightward by the fixed vertical clutch plate 210 of Fig. 7, causing the hook bar 120 to be rotated about the hinge pin 121 in the counterclockwise direction, so that a hook protrusion 122 is disengaged from the hook hole 131. As a result, the landing doors 100 are opened to a state indicated by dash dot dotted lines in Fig. 5. As such, when the first push roller 125 is pushed rightward, the landing doors 100 are opened to both sides. The reason is that, since the landing door holding plate 110 of the right side is fixed below the wire rope 105 and the landing door holding plate 110 of the left side is fixed above the wire rope 105, the wire rope 105 is moved in the counterclockwise direction to open the landing doors 100. After the initial opening operation of the elevator door system as described above, the opening operation of the door system is changed from the state shown in Fig. 7 to the state shown in Fig. 8 at a middle opening stage of the door. That is, as the cam plate lever 260 comes near to a vertical location of the switch shaft 262, the cam plate lever 260 is rotated about the cam hinge pin 253 in the coun¬ terclockwise direction, so that the clutch plate lever 220 is rotated about the clutch plate lever hinge pin 214 in the counterclockwise direction while the cam roller 240 contacts the second cam groove 252. Since the clutch plate lever 220 is movably fastened at the center thereof to the clutch plate lever supporting plate 230 (car door holding plate 201) by the clutch lever hinge pin 214, and is also movably fastened at the lower end to the vertical clutch plate 211, the vertical clutch plate 211 is separated from the clutch plate 210 as shown in Fig. 8. As a matter of course, an extent of separation corresponds to the gap between the first and second push rollers 125 and 142 shown in Figs. 5 and 6. [57] The present invention is characterized in that the doors are reduced in closing speed after being opened to a predetermined extent as shown in Fig. 8 in order to ensure passenger safety, thereby preventing the passenger from being trapped between the doors. For this purpose, the elevator door system comprises the acceleration-reduction cams C as shown in Fig. 11. Each of the acceleration-reduction cams C has a slant cam section 278-5 at an interface between a large diameter section 278-4 and a small diameter section 278-3 of the cam plate 278 to drive an associated limit switch 276 having a movable load 276-1 connected to the slant cam section 278-5. As a result, as shown in Fig. 11, the acceleration-reduction cams C serve to detect an open position, a close position, and an acceleration-reduction position with different phases, re- spectively. For example, when an end of the large diameter section 278-4 of each ac¬ celeration-reduction cam Cl contacts an associated movable load 276-1 as shown in Fig. 11, an associated limit switch 276 is operated to allow the controller (not shown) to recognize the position of the acceleration-reduction cam as indicating a closed state of the door. Likewise, a position of an acceleration-reduction cam C2 must be recognized as indicating a command for closing deceleration immediately before the doors are closed (in this case, as with the acceleration-reduction cam Cl, the end of the large diameter section 278-4 of the acceleration-reduction cam C2 contacts an associated movable load 276 of the associated switch SW2), a position of an ac¬ celeration-reduction cam C3 must be recognized as indicating a command for opening deceleration, and an acceleration-reduction cam C4 must be recognized as an open state of the door. For this purpose, the acceleration-reduction cams C are equipped on the shaft 262 with different phases. [58] In particular, the present invention has an advantage in that, in each of the ac¬ celeration-reduction cams C, the tap plate 277 formed with the radial tap holes 277-1 contacts the cam plate 278 formed with elongated holes 278-1 such that the phase angle (positions of the large diameter section 278-4 and the small diameter section 278 -3) of the cam plate 278 can be easily adjusted, and maintained by the control screw 278-2 through the holes. Accordingly, when the car door is opened, the cam plate lever 260 shown in Fig. 7 is rotated in the clockwise direction, and allows the switch unit 275 to be rotated in the counterclockwise direction (in this case, the cam plate lever 260 is integrally operated at the outside (right side) of an upper end of the switch shaft 262 of Fig. 11), and, in the state shown in Fig. 7, the end of the large diameter section 278-4 of the acceleration-reduction cam Cl of Fig. 11 pushes the movable load 276-1 of the associated limit switch 276 to turn on the controller. Additionally, as shown in Fig. 8, when the elevator entrance is in a state immediately before being completely opened, the end of the large diameter section 278-4 of the acceleration-reduction cam C3 of Fig. 11 pushes the movable load 276-1 of the associated switch SW3 to operate the associated limit switch 276 (in Fig. 11, it can be appreciated that, although not in a pushing state, the acceleration-reduction cams C are operated with the same phase of the rotation of the switch shaft 262). Then, the controller (not shown) is turned on to detect this state, and sends a reduction command to the motor 271 shown in Fig. 8, so that the switch unit 275 is closed while the car door is decelerated. [59] When the door is completely opened as shown in Fig. 10, the switch shaft 262 is rotated, and the switch unit 275 in the closed state shown in Fig. 11 is operated such that the end of the large diameter section 278-4 of the acceleration-reduction cam C4 of Fig. 11 pushes the movable load 276-1 of the associated switch SW4 to operate the associated limit switch 276. Then, the controller detects complete opening of the door, and stops the motor 271. Meanwhile, according to the invention, the counterweight 262 is provided on the upper end of the cam plate lever 260, and provides an assistant function to reduce power of the motor 271 at initial and final stages of opening or closing the door, thereby allowing smooth opening/closing while enhancing endurance of the elevator door system. [60] According to the invention, a closing operation of the elevator door system is also performed according to the same operating principle. That is, a closing speed of the doors is low at an initial closing stage shown in Fig. 10, and increased at a middle closing stage. When the door are in the state shown in Fig. 8, the vertical clutch plate 211 pushes the second roller 142 to close the door, and, as the cam plate 250 is operated to reduce the distance between the clutch plates 210 and 211, so that a pushing speed of the clutch plate 211 against the second push roller 142 is naturally reduced. According to the invention, the switch unit 275 is operated in conjunction with interconnection between the cam plate 250 and the cam plate lever 260 from the state shown in Fig. 8, so that the end of the large diameter section 278-4 of the ac¬ celeration-reduction cam C2 of Fig. 11 pushes the movable load 276-1 of the associated switch SW2 of the limit switch 276. As a result, the controller reduces the speed of the motor 271 to reduce a final closing speed of the door, thereby preventing accidents such as trapping of fingers between the closing car doors 200. In this case, when closing the landing door 100, tension of the tension spring 113 is applied to the landing door 100 through the return load 112, so that the landing door 100 is naturally closed by the roller 110. Thus, the landing door 100 is closed without additional power. When the landing door 100 is completely closed, the hook protrusion is hooked onto the hook hole 131 of the hook coupling plate 130, and holds the landing door to the landing floor frame 103, thereby preventing the accidents. [61] When closing the car door 200, the assistant stopper 290 contacts the stopper roller 293, and acts as a stopper. In this regard, since the assistant stopper 290 comprises angled cams 291-1, 291-2 and 291-3, and thus provides repetitious friction as with an ABS system of a brake in a vehicle, it can act as a smooth and strong stopper. This function can be achieved since the roller holding plate 294 supporting the stopper roller 293 supplies the resilient force to the stopper roller 293 around the fixing plate hinge 295 provided at the center of the roller holding plate via the spring 297 provided to the other end thereof. When the door is completely closed, a closing switch pusher 286 pushes and turns on the closing switch 287 to completely stop the motor 271. [62] Although the preferred embodiments of the invention have been disclosed for il¬ lustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.