To rapidly and effectively conduct the clamping operation with a simple structure.

A horizontal guide 3 is provided, and a pair of right and left vice pieces A, B are movably provided at the guide 3. Upwardly urging means E for urging the pieces A, B upward are provided at the pieces A, B. Link arms 6a, 6b coupled at the one ends to the pieces A, B and opposed at the other ends to the center of switching are disposed, and the piston rod 8a of a hydraulic cylinder 8 vertically operating is coupled to the other ends of the arms 6a, 6b.