PURPOSE: To enable securely holding a heavy container and prevent a excessive deforming of a flexible container by providing rocking parts and a stopper, the rocking parts being provided on the side of clamp shafts and clamp arms respectively and designed to act in conjunction with them and the stopper being designed to stop the arms from acting during their closing action.

CONSTITUTION: Reinforcing rods 37A, 37B are fixed respectively to the forward ends of rocking arms 36A, 36B which project radially from the lower ends of a pair of clamp shafts 26A, 26B. One clamp arm is attached to the clamp shaft 26B and the reinforcing rod 37B and another clamp arm is attached to the clamp shaft 26A and the reinforcing rod 37A. The rocking arms 36A, 36B provided in a pair face parts partly forming rocking parts 36a, 36b which spread radially outward with a stopper 39 disposed therebetween. While the two rocking arms 36A, 36B, that is, the two clamp arms, are in rocking motion for the closure from the full-open position to the full-closed, the stopper comes in touch with the rocking parts 36a, 36b and stops them from their closing action.