PURPOSE: To improve the feeling of turning operation, startability or the like by combining an elastic member with a pressure rise prevention section capable of reducing capacity, depending upon the turning angles of a seat and a seat cover, and providing a safety valve.

CONSTITUTION: A pressure cell section 15 moves in space 12 and oil flows through a small gap between the section 15 and a case 2. Also, as a groove section 28 moves toward a collapsing direction, an oil flow amount is gradually reduced due to a flow through the a shallow groove, thereby improving the feeling of turning operation. When the turning operation further progresses, the section 15 comes in contact with a slider 24 and the elastic force of a coil spring 22 acts, in addition to the pressure of the oil, thereby giving a further braking effect. Also, when the internal pressure of the case 2 rises to constant level or higher, a safety valve 19A is opened to restrain an excessive load. The second pressure rise prevention section easily opens a reducing valve 17A having a weak spring force, due to the turn thereof to an erecting position O, thereby discharging oil from space 12 positioned at the erect position O. According to this construction, turning resistance in the erect position becomes smaller than turning resistance in a collapsing direction. Thus, a seat cover and a seat can be returned to an open position lightly and smoothly.