An engagement fixture of a shoulder belt is connected to and supported by an engagement buckle disposed in a vehicle interior. When a platform is positioned in a getting in/out position in a vehicle exterior, the engagement fixture of the shoulder belt can be temporarily connected to a temporary fastening fixture provided to the platform. To the contrary, when the platform is positioned in a loading position in a vehicle interior, a connection condition of the engagement fixture of the shoulder belt can be changed from a temporary connection of the fastening fixture to a securely supportable connection of an engagement buckle in the vehicle interior. Thus, it is possible to simplify a belt attaching operation after a wheelchair is loaded in a vehicle while a columnar support member used in the conventional platform is abolished.