To reduce torque required for a turn of a hand crank even in a clamping process by providing an epicyclic gearing between a clamping member and a driving shaft, and providing a spring loaded locking coupling between a casing and the clamping member.

When a clamping member 14 is rotated through an epicyclic gearing 30 by a hand crank mounted to a rear end 28a of a driving shaft 28, since the clamping member 14 is connected out of rotation to a casing 10 by a locking coupling 38, a screw spindle 9 is rotated first, and a clamping jaw at the tip is brought into pressure contact with a workpiece by the axial movement of the screw spindle 9. When clamping torque exceeds torque transmittable by the locking coupling 38, a locking member 39 is pushed radially outward so as to come off a groove 42. Then the clamping member 14 is rotated relatively to the casing 10 to make a clamping pin 15 stand up, a clamping member 13 is slided to the left, and a slider 3 presses the workpiece with large pressure.