To enable smooth and accurate horizontal movement of a slider which is advanced or retracted being guided by a support member while facilitating the mounting of members forming an advancing/retracting drive mechanism X and also achieve a manufacturing at a lower cost.

Support members 5 are arranged horizontally on a floor surface 3 in a room of a vehicle and an advancing/retracting drive mechanism X for a platform 4 equipped with slider arms 14 that are allowed to advance or retract through respective guide members is mounted on the support members 5. Each one end part of upper and lower parallel moving arms 31 and 33 forming a 4-joint link in combination with the slider arms 14 and a roller support plate 36 is pivoted at protruded tip parts of the slider arms. In addition, a lift drive mechanism Y is mounted to lower the platform 4 completely protruding from the floor surface 3 almost as intact downward maintaining its horizontal state or to lift it as intact maintaining its horizontal state from the lowering position.