To simplify a supporting structure more and to make turning stepless by connecting and fixing a cylinder piston mechanism to the rear end of a seat surface for which a backrest is synchronously turned.

A projected surface 5 whose cross section is turned to a semicylindrical shape is provided on the bottom surface of the seat surface 4 and the seat surface 4 is pivotally supported to a supporting base 1 turnably by fitting a shaft pin 6 to a supporting hole 3. Also, the seat surface 4 and an armrest 8 independent of the backrest 7 are attached near the upper end of the supporting body 2 of the supporting base 1 through a supporting column 9 using a pipe body and the backrest 7 is integrated with the seat surface 4 and synchronously turned. At the rear end of the seat surface 4 positioned right below the backrest 7, the end part of a piston 11 to be slid inside a cylinder 10 is pivotally supported to a supporting flange 12 provided on the seat surface 4 and the cylinder 10 is pivotally supported to the supporting flange 13 provided on the supporting base 1. A lever 14 disposed under the armrest 8 is connected to a wire 15 passed inside the supporting column 9, a valve is opened by the pulling action and the piston 11 is driven.