An output part is provided to the left end of a clamp arm that is supported by a housing so as to be slidable in the vertical direction. At the left end of the housing, a first hole for spring fitting opens obliquely upward towards the output part. A through hole that links the upper surface and the lower surface of the clamp arm is disposed obliquely upward at a left-leaning part of the clamp arm. A second hole for spring fitting and a female screw hole are formed, in that order, upwards in the through hole. A return spring that is installed through the first hole and the second hole impels the output part upwards. A push bolt that is screwed into the female screw hole rom above compresses the return spring.