To improve workability to grasp or release a body to be grasped.

A vertical hole 20 approximately orthogonal to an oil passage 16 and opened to an outer peripheral surface is formed and a plunger 24 is fitted in the vertical hole 20. The vertical hole 20 comprises an oil chamber 21 communicating with the oil passage 16; a large diameter hole 22 positioned on one side of the oil chamber 21; and a small diameter hole 23 positioned on the other side of the oil chamber 21 and having diameter smaller than that of a large diameter hole 22. The plunger 24 comprises a large diameter part 24a fitted in the large diameter hole 22; a small diameter part 24b fitted in the small diameter hole 23; and a taper part 24c intercocommunicating the large diameter part 24a and the small part 24b and positioned on the large diameter hole 22 side of the oil chamber 21. An operation ring 27 is rotatably positioned on the outer peripheral surface of a clamp body 1, and the center of the inner peripheral surface of the operation ring 27 is positioned eccentrically from the center of the outer peripheral surface to the large diameter part 24a side of the plunger 24. When the operation ring 27 is rotated, the inner peripheral surface 32a of an inner race 32 presses the plunger 24, and the taper part 24c of the plunger 24 pressurizes hydraulic oil in the oil chamber 21.