<p>A clamp rod (6) inserted into the guide hole (5) of a housing (4) is provided with a clamp member (10), an inner piston (12) and a tapered pressure receiving surface (13) sequentially from the left end toward the right end in the central axis direction. An output portion (26) for lock and release is provided on the inner circumference at the right end of one outer piston (23) inserted between the inner piston (12) and the guide hole (5). A collett (33) inserted between the pressure receiving surface (13) and the guide hole (5) is constituted of four split members (34) arranged in the circumferential direction at substantially regular intervals. An input portion (36) driven by the output portion (26), and a pressing surface (40) making wedge-engagement with the pressure receiving surface (13) from the right side are provided, respectively, at the outer circumferential part and the inner circumferential part of each split member (34). A lock chamber (42) being supplied with pressure fluid and a release chamber (47) for discharging pressure fluid are provided on the outside of the both ends of the outer piston (23).</p>
