To simplify switching mechanism between rapid traverse and temporary clamping and proper clamping operation so as to reduce manufacturing cost with simple constitution.

A fixed clamping part 2 with a first clamping face 21 coming in contact with a workpiece 4 is provided on a base 1, and a movable clamping part 3 with a second clamping face 31 opposed to the first clamping face 21 is movably provided on the base 1. A screw member 5 with an external thread provided at the periphery so as to move the movable clamping part 3 is provided and disposed on a semicircular first guide groove 61 without a thread part of a fixed block 6. An opening/closing block 7 is placed on top of the fixed block 6, and this block 7 has a semicircular second guide groove 71 with an internal thread in a position opposed to the first guide groove 61. At the time of rapid traverse, the opening/closing block 7 is opened to slide the screw member 5 by the first guide groove 61. At the time of temporary clamping and proper clamping, the opening/closing block 7 is closed and fixed by a connecting pin 9, and the screw member 5 is turned in a clamping direction by the second guide groove 71 to clamp the workpiece 4.