MAILING ENVELOPE Technical Field [1] The present invention relates to mailing envelopes, and more particularly, to a mailing envelope which can be easily opened without damaging the mailed material contained therein and maximally guarantee the addressee a private secret because once opened it cannot be restored to its original shape. [2] Background Art [3] Generally, a mailing envelope is classified into a small-sized envelope for enclosing a letter or brief dociments, a middle-sized envelope for enclosing a variety of invitation cards etc., and a large-sized envelope for enclosing brochures or documents containing several tens of sheets of paper. Although the mailing envelope is different in size according to its purpose of use, it typically has a rectangular shape and includes a front panel on which addressee's address is written, a back panel forming an envelope body by connecting three surfaces extended foldably from the front panel to one another, and a closure flap for sealing it. The front panel and the back panel are integrally connected through three surfaces. An opening is formed between the front panel and an open edge of the back panel so as to insert a letter or dociments thereinto. The closure flap is extended vertically from the front panel and folded to be overlapped with the open edge of the back panel. After a letter or dociments is inserted into the opening, the envelope is supposed to be sealed by gluing the closure flap and the back panel together. [4] For the conventional mailing envelope, in order for the addressee to see the contents of a letter, he or she should tear off a part of the envelope open with his or her hands along a fold line by which the closure flap and the front panel intersect, or cut an end of the envelope by using small tools such as scissors or a paper knife. Therefore, the opening operations are cumbersome, and sometimes a letter or documents enclosed in the envelope is torn off or cut together with the envelope. [5] To solve these problems, improved envelopes of various forms have been proposed. For instance, envelopes are provided with perforated cutting lines formed at their ends in the form of dotted lines. These envelopes are opened along the cutting lines. Although such envelopes facilitate their opening operations, since the cutting lines are very weak, they still embrace the risk of damaging the cutting lines during the mail treatment and damaging the mailed material together with them. Envelopes are also provided with thread or a tape buried thereto. If a user pulls the thread or tape, its peripheral portion is torn off and simultaneously the envelope is opened. However, since these envelopes have high manufacturing cost, they are not suitable for the existing small- or middle-sized envelopes. [6] In addition, the conventionally improved envelope has a structure to open not its sealed closure flap but other parts except the closure flap. Therefore, even if someone stealthily reads a letter by opening the sealed part of the closure flap and then seals it again, the addressee may not recognize the fact, failing to fully guarantee his or her private secret. [7] Disclosure of Invention Technical Problem [8] Therefore, the present invention has been made to solve the above problems occurring in the prior art, and it is an object of the present invention to provide a mailing envelope which can be easily opened without damaging the mailed material contained therein and maximally ensure a guarantee of addressee's private secret because once opened it can't be restored to its original shape. [9] Technical Solution [10] To accomplish the above object, according to the present invention, there is provided a mailing envelope including a rectangular front panel on which addressee's name and address are written, a back panel forming an envelope body by connecting three surfaces extended foldably from the front panel to one another, and a closure flap extended vertically from the front panel and adapted to fold along a fold line to be overlapped with an open edge of the back panel. The back panel has a cut strip of a given width formed along the open edge thereof, the cut strip having an identically repeated shape of cuts. An open edge of the closure flap is laid over the cut strip when the closure flap is folded toward the back panel, so that if the closure flap is opened under the state in which the closure flap adheres to the back panel, the cut strip of the back panel is torn off along the cuts. [H] Brief Description of the Drawings [12] Further objects and advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: [13] FIG. 1 is a perspective view illustrating a mailing envelope according to an embodiment of the present invention; [14] FIG. 2 is a perspective view illustrating the mailing envelope of FIG. 1 in use; [15] FIG. 3 is a perspective view illustrating a mailing envelope according to another embodiment of the present invention; and [16] FIG. 4 is a perspective view illustrating a mailing envelope according to a further embodiment of the present invention. [17] Mode for the Invention [18] Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. [19] Referring to FIG. 1, a mailing envelope according to the present invention includes a rectangular front panel 10 on which addressee's name and address are written. A back panel 30 forms an envelope body by connecting three surfaces extended foldably from the front panel to one another. A closure flap 12 is extended vertically from the front panel and folded to be overlapped with an open edge of the back panel 30. A cut strip 20 having a given width is formed along the open edge of the back panel 30 by discontinuous cuts or incisions 22. The cut strip 20 has an identically repeated shape of the cut 22. [20] The cuts 22 may have a horseshoe shape as shown in FIG. 1, a comb-tooth shape in FIG. 3, or a saw-tooth shape in FIG. 4. In addition, the cuts 22 may have a scale-like shape( ))))))))) ). It is preferable for the cuts 22 to have a directional shape like the comb-tooth or scale-like shape. For example, the cuts 22 slanted from the right top to left bottom as shown in FIG. 3 are very effective when a user opens an envelope from the right to left. When the closure flap 12 is folded toward the back panel 30, the open edge of the closure flap 12 is extended over the cut strip 20, so that a part of the cut strip 20 is hidden by the internal surface of the closure flap 12 and the other part of the cut strip 20 is exposed to the outside with a predetermined width along the open edge of the closure flap 12. Therefore, if a user opens the closure flap 12 under the state in which the closure flap 12 adheres to the back panel 30, the cut strip 20 of the back panel 30 is ripped off along the cuts 22 together with the closure flap 12. [21] As illustrated in FIGs. 3 and 4, a lead slit 24 for connecting an end portion of the cuts 22 with the open edge of the back panel 30 may be formed at one end or both ends of the cut strip 20. The lead slit 24 is helpful to easily tear off the cut strip 20 along the cuts 22 when the closure flap 12 is opened. [22] Meanwhile, an adhesive zone 40 is formed in a usual adhering manner on a portion of the inner surface of the closure flap 12 which is to adhere to the cut strip 20. Hence, the envelope can be easily sealed without additional adhesives. The adhesive zone 40 may use solid adhesives glued with only water or thin film adhesives glued by peeling out a thin film. There is formed a graspable tab 16 extruding from one end of the closure flap 12 to facilitate the opening of the closure flap 12. [23] Industrial Applicability [24] As described above, the inventive envelope can be easily opened without damaging the mailed material contained therein. Once the envelope is opened, it can't be restored to its original shape. Therefore, an addressee's private secret can be maximally guaranteed. [25] While the present invention has been described with reference to the particular il¬ lustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.