A left end portion of a clamp arm is vertically swingably connected to an upper portion of a rod vertically movably inserted into a housing, and an annular supported member is attached to a support member rotatably around an axis, the support member being provided to an outer periphery of an upper portion of the housing. A pivot portion provided to the supported member and a non-edge portion in a longitudinal direction of the clamp arm are connected by a link member. The support member and the supported member are provided with an element for receiving an upward force acting from the link member on the pivot portion during a clamping operation. The supported member is fixed to the support member at a desired rotation angle position by a lock element.