PURPOSE: To easily open and close a door body by moving the door body with a motor to open and close an opening, obviate a clamping device and enhance the opening and closing controllability to reduce the cost.

CONSTITUTION: In the case where a door body 1 is closed, a gear 19 is rotated and driven with a motor 17, and hanging pulleys 7 and 8 are rolled on rail sides 11 and 12 of a rail 9 to move the door body 1 in the closing direction. The gear 19 descends a slope of the broad section 22 of a chain 21 at a closing position, the hanging pulleys 7 and 8 fall into a recess 15 of a slope section 14 of the rail sides 11 and 12 by the own weight of the door body 1. At the same time, the gear 19 is so moved that it slides on the inside of the broad section 22 of the chain 21 while pushing the door body 1 to the inside with the own weight by projected size and length of the slope section 14, and an opening 3 is closed. In the case where the door body 1 is opened, the gear 19 is so moved that it slides on the broad section 22 of the chain 21 to ascend a slope while interlocking the hanging pulleys 7 and 8 with the upper edge of the slope section 14 from the recess 15 of the rail sides 11 and 12. The door body 1 is moved to the front side while moving it to the front side, and the opening 3 is released.