To make it possible for the user to use a seat comfortably by allowing a backrest to be adjusted in an angle so that its top may be inclined most deeply backward in the height direction from the center of its width.

The user sits on a seat and operates a lever 41 of an operating section 40 installed on an armrest E to let a controlling section 50 operate. A fluid pump 30 is started to operate at a command signal from the controlling section 50. By supplying pressurized fluid to a left fluid cylinder 20L and backing a piston lever 21L, the top of a left movable supporting frame 122L is backed. Thereby, the left movable supporting frame 122L is so displaced as to be inclined backward with respect to a fixed supporting frame with a hinge as a fulcrum and at the same time, a part of a backrest installed on the front face of the left movable supporting frame 122L that is near the top of the backrest that supports the left shoulder of the user is also inclined backward. When the user uses the seat with its back abutted against the backrest under this condition, the user can sit on the seat stably and comfortably for a long time.