this time the support blocks 75 at the two end portions of the rod 73 are supported movably relative to the support rollers 77 as has been described hereinbefore the rod 73 is reciprocated in accordance with the reciprocal movements of the cylinder tube 69 relative to the rod 73 thereby to turn the wrench member 65 reciprocally on the socket portion 67 as a result the nut member 61 meshing with the socket member 67 is turned in the fastening direction toward the stationary and movable mounts 11 and 15 thereby to thrust the leading end portion of the clamping plate 55 to the mold bed 81a or 83a since at this time the nut member 61 meshes with the support bolt 51 through the bearing 63 the nut member 61 can be fastened with a low torque because its contact resistance with the clamping plate can be reduced when the fastening force of the mold bed 81a or 83a by the clamping plate 55 and accordingly the feed resistance of the air to the cylinder tube 69 exceeds a predetermined pressure the feed of the air to the cylinder tube 69 is interrupted to complete the clamping operation in the description thus far made the wrench member 65 is turned reciprocally on the socket portion 67 by causing the support blocks 75 attached to the two end portions of the rod 73 to abut against the support rollers 77 thereby.