PURPOSE: To reduce the number of parts, and to reduce manufacturing cost by providing a taper part at least on either one side of the outer circumference at one end of a strut or the inner circumferential surface of a case so that the inside hole diameter of a piston of a clearance S is reduced on the bottom side.

CONSTITUTION: A taper part 21c is provided at least on either one side of the outer circumference at one end of a strut 20 or the inner circumferential surface of a case 21 so that the inside hole diameter of a piston 15 of a clearance S is reduced on the bottom side. Each ball 22 is energized to the left by a first spring 23, and the case 22 is energized to the right by a second spring 24. In the ordinary time, each ball 22 is engaged with the enlarged diameter part 20d of the strut 20 and the taper part 21c by the wedge action between these parts. Thus, the number of parts can be reduced, and the reduction in manufacturing cost can be achieved.