To miniaturize a damper housing, to reduce its weight, and to enhance the sealing performance of a sliding part.

A damper housing 2 is provided with an arc-shaped cylinder 9, which is concentric with an input/output shaft 3. The input/output shaft 3 is provided with an arm 18, which extends from the shaft 3 toward the outside in the radial direction, as an integrated body, while the arm 18 is connected to an arc-shaped piston rod 17, which is concentric with the input/output shaft 3. The piston rod 17 is fitted into a cylinder tube 10 in a freely retreatable manner, while the piston rod 17 is provided with a piston valve 22, which is fitted into the cylinder tube 10 in a freely slidable manner. Rotataion of the input/output shaft 3 relative to the damper housing 2 rotates the arc-shaped piston rod 17 together with the input/output shaft (as an integrated body) via the arm 18, so that the piston valve 22 moves within the arc-shaped cylinder tube 10, thereby generating the damping force.