PURPOSE: To prevent a door end from descending at the time of using a sliding type door as swinging type by disabling the horizontal travel of a swinging door shaft through a locking mechanism.

CONSTITUTION: When an open/close door 58 is used as a sliding door, the lower end of a descending member 4 comes in contact with the top rail 58a of the door 58, thereby pushing down the right end 5a of an oscillation member 5. Subsequently, an arm 6 is also pushed down and a gap B between the upper belt 1 of an annular door drive belt 52 and the upper end 6b of the arm 6 is properly kept, thereby causing the door 58 to travel along a rail 61 driven by the belt 52. Also, when the door 58 in use as sliding type is pushed, the operation piece 60a of a latch 60 is disengaged from the rail 58a, and this disengaged state is detected to interrupt power supply to a belt drive source. As a result, the lower end of the descending member 4 is disengaged from the rail 58a and comes down. At the same time, the right end 5a of the member 4 ascends, thereby causing the arm 6 to ascend as well. Also, the gap B narrows and the belt 52 is clamped between the upper end 6b of the arm 6 and the upper end 7b of a fixing member 7. As a result, the belt 52 stops travelling and the horizontal travel of the door 58 along the rail 61 is disabled. According to this construction, slidable contact between a door end and a floor can be prevented.