PURPOSE: To easily set a clamp head to be fixed by a second part of a connection link guide at a clamping location by providing a guide plate provided on a sliding part of a clamp block freely movably in the longitudinal direction within a guide groove of a yoke plate and mounting the clamp head freely movably in the longitudinal direction on this sliding part.

CONSTITUTION: In a state that a guide plate 12 passes through a guide groove 6 existing within the longitudinal direction center of a yoke plate 2 and is engaged, a clamp block 7 is held by a sliding plate 30 and the sliding plate 30 is located on the lower side of a base part 1. This sliding plate 30 is sunk within a groove 31 on the lower side of the yoke plate 2 and all these parts are caved in yoke plate bottom plates 2, 1. At this time, the sliding plate 30 is divided into two longitudinal direction parts by a groove 32 extending in the horizontal direction and the sliding plate 30 is fixed to the bottom surface of the guide plate 12 by two hexagonal socket screws 35 on a mounting part 33. Therefore, the clamp block 7 can be moved with hardly having play between fixed jaws 3 on the base part 1.