PURPOSE: To regulate an attenuation force generated, by operating a rotary valve in a bypass path affording communication between the upper and lower chambers in a cylinder to change an oil amount in a path outside the cylinder affording communication between the upper and lower chambers.

CONSTITUTION: Upper and lower chambers A, B are provided in a cylinder 10, and a path (a) affording communication between the upper and lower chambers A, B is provided outside the cylinder 10. Also, a bypass path R affording communication between the upper and lower chambers A, B is formed in the lower end of a piston rod 21, and a rotary valve 50 is disposed in the bypass path R. Further, an actuator 54 is mounted in a housing 53 fixed to the upper end of piston rod 21, and when a cross hole 21b is opposed to a lower longitudinal groove by pivoting a main body, the bypass path R is brought to the communicating condition by the rotary valve 50 so that a generated attenuation force is adjusted by pivoting the actuator 54.