To prevent the gap between an oscillation arm and a platform from being increased by providing a spring to urge a base end of the oscillation arm in the direction of an edge corner part and an expandable film to be stretched between a tip of the oscillation arm and a part in the vicinity of the corner part.

A roller 17 is rotatably mounted on a base end of an oscillation arm 16. The base end of the oscillation arm 16 is movable along a vehicle connection surface 1. A spring 18 in the compressed condition is mounted on a center side of the vehicle connection surface 1 of a guide rail 15, and the base end of the oscillation arm 16 is urged in the direction of an edge corner part 6. A tip of a turning arm is pivotably attached by a pin 20 approximately to a center of the length of the oscillation arm 16. A longitudinal frame 23 is fixed to a side a vehicle side surface 5 of a bracket 21. A longitudinal frame 24 is also fixed to a tip of the oscillation arm 16 extending to the position close the extension surface of the vehicle side surface 5. An expandable film 25 of bellows shape is stretched between the longitudinal frame 23 and the longitudinal frame 24.