PURPOSE: To relieve or check the swing of backrest supporting lever with the effect of gas spring in locked status when additional backward pression is applied to the backrest supporting lever under the backward tilting of user.

CONSTITUTION: A backrest supporting lever 4 is connected to a fixed frame 1 on seat side such that the backrest supporting lever 4 is durable for the backward tilting, base of cylinder 11a at gas spring 11 is connected to the backrest supporting lever 4, the point of a piston rod 15 moving to the opposite side of cylinder 11a is connected to the fixed frame 1 through a bracket 13. A push rod 16 at the point of piston rod 15 is controlled by applying pressure to control lever 20 to switch gas spring 11 to longitudinally movable status and to locked status and to keep the backrest supporting lever at predetermined tilt angle. A tube 17 fixed on the bracket 13 is mounted so as to be movable with respect to pheripheral surface of the cylinder 11a under the presence of slide resistance and the swing is relieved or checked by the potensiometer.