PURPOSE: To promote the compactness of a device by reducing the sliding direction dimension of a piston member and avoid the generation of a stick slip.

CONSTITUTION: A ring-like piston member 38 is set rather small in the inner and outer diameters so that the outer peripheral surface and inner peripheral surface of the piston member 38 do not come in contact with the outer cylinder 36b and inner cylinder 36a of a ring member 36 when the piston member 38 slides in a piston storage chamber 36d. Inside the piston storage chamber 36d, the piston member 38 is elastically supported to the cylinder member 36 through elastic seal members 45, 46 mounted on the inner and outer peripheries of the piston member 38 so as to secure clearances S respectively between the outer peripheral surface and the outer cylinder 36b of the cylinder member 36 and between the inner peripheral surface and the inner cylinder 36a of the cylinder member 36.