To achieve back aisle when seated without impairing freedom in deign in order to secure back aisle when seated and convenience of seating and standing.

An operating member 2 is located near the base 1 installed on the floor and a four-point link mechanism 3 is formed between the operating member 2 and the base 1. On the operating member 2, at least the base edge 5a of the seat supporting member 5 is mounted and the seat 6 in a seated position is moved forward and backward in approximately horizontal direction by the operation of the four-point link mechanism 3, when the seat supporting member 5 is moved.