A shock absorber (D1) comprises a ring-shaped seal member (3) supported by a cylinder (1) so as to cause a tip to contact the piston rod (2) resiliently. A ring-shaped actuator (4a) exerts an inward force on the outer circumference of the seal member (3). The actuator (4a) is constituted by a polymer material that varies its diameter upon impression of a direct-current voltage. By controlling the direct-current voltage depending on the relative displacement speed between the piston rod (2) and the cylinder (1), the frictional resistance which the seal member (3) exerts on the piston rod (2) is kept large when the relative displacement speed is in an extremely low operation speed region, and is decreased as the actuator (4) decreases the inward force when the relative displacement speed increases above the extremely low operation speed region.