PURPOSE: To maintain the door closed state, preventing the lowering of the door even if air pressure is reduced when the door is in closed state, by installing, in the lower part of the getting-on and -off door, an engaging-rod receiving part which projects in the horizontal direction from the lower part of a chassis floor by the air pressure.

CONSTITUTION: A rotary shaft 4 is installed onto a chassis 3 side along the side edge in the longitudinal direction of the getting-on and -off door of a bus, and a door opening and closing cylinder 5 is installed at the edge part. A cylinder 16 for locking is installed in the horizontal direction in the lower part 15 of a chassis floor which is opposed to the lower part 2b of the door 2, and an engaging rod 18 which projects in the horizontal direction when the cylinder 16 is operated by the application of air pressure is installed. An engagement hole 2c with which the rod 18 is engaged when the door 2 is set into closed state is formed in the lower part 2b of the door 2. When the door 2 is in closed state because of the parking for a long time, the cylinder 16 for locking is operated, and the engaging rod 18 is engaged with the engagement hole 2c, and the door closed state is maintained.