PURPOSE: To clamp a member to be clamped under a prescribed pressure by providing a supporting member which movably supports a clamper in a clamping direction, a spring which forces the clamper to a clamp member side, and a sensor which outputs a signal at a prescribed position.

CONSTITUTION: When a motor 55 rotates, a screw shaft 53 rotates until a clamper 31 which is supported on a supporting member 35 supported with a body part 51 comes into contact with a member to be clamped 33 with the supporting member 35. When the supporting member 35 moves to the member to be clamped 33 side further, the clamper 31 moves relatively with regard to the supporting part 35 against the forcing effort of a spring 43. When the upper edge of the clamper 31 relatively moves to a prescribed position, that is, a sensor 49 side position upper than the supporting part 37 by a distance L set in advance with regard to the supporting member 35, a stopping signal is outputted to a controller by a sensor 49 to stop the operation of a motor 55. Pressure to clamp the member to be clamped 35 can be changed easily by changing the elastic coefficient of the spring 43 or the fixing position of the sensor 49.