PURPOSE: To generate a high damping force through a specified rotation stroke or more by forming a ball guide groove, having a deepmost depth and a slope, in a surface where a rotor and a piston housing are positioned facing each other, locating a ball having size larger than a deepmost depth, and filling a chamber with viscous oil.

CONSTITUTION: When a rotary shaft 3 is rotated in a state that a rotor 12 is located in a neutral position, the rotor 12 is rotated in the same direction as that of the rotary shaft, the volume of a chamber S1 or S2 in a rotation direction is reduced, high viscosity oil in the chamber is forced to flow out through a gap 20 to the opposite chamber S2 or S1, and an internal pressure is generated in the chamber S1 or S2. Though an internal pressure is gradually increased, provided a rotation speed is set to a specified value, a ball 30 is rolled in the deepmost surface part 40a of a ball guide groove 40 in a rotation stroke range. The press force of a friction clutch 4 by a piston 9 is limited and a specified damping force is generated. At a specified rotation stroke or more, the ball 30 is rolled on the slope part 40b of the ball guide groove 40, the piston 9 is pressed by the ball 30, and the friction clutch 4 is pressed. This constitution increases the generation of a friction force and generates a high damping force.