PURPOSE:To prevent deterioration of operability by dividing a cylindrical chamber into two liquid chambers by means of a piston, communicating the liquid chambers with each other by means of a through-hole provided with a needle valve, and stopping a throttle mechanism of the needle valve at a specified position at the time of a low temperature. CONSTITUTION:A shock torque preventive valve 6 is arranged between a master cylinder 4 and an operating cylinder 8. A piston 16 is fitted to a hole 12b of a cylinder body 12, while a valve body 14a of a needle valve 14 is fitted to a through-hole 16a. When a temperature of operation liquid exceeds a specified value, a ring 22 made of memory shape alloy is set at an unoperational position as indicated by a figure. When the operation liquid is not more than the specified value, the ring 22 is deformed inward, preventing the motion of a spring 18 and the leftward motion of the piston 16 as a stopper. Throttle of a flow passage 14b is provided slightly, thereby it is possible to prevent deterioration of operability of the clutch, and to prevent clutch abrasion.
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