PURPOSE: To improve workability by pressing a lock piece to a drum with a piston to lock an upper main body, and releasing the pressing force of the piston to return the lock piece with the spring pressure.

CONSTITUTION: When a drum 2 is to be stopped at a preset angle, a hydraulic unit is operated, the operating oil is fed to a hydraulic cylinder 7 to extrude a piston 8, a lock piece 6 is extruded to the outer periphery side of the drum 2, and the drum 2 is fixed at an optional position. If the cylinder 7 is sealed while the operating oil is fed, the pressed state to the outer periphery of the drum 2 by the lock piece 6 is held, and the drum 2 is locked and fixed. When the drive of the hydraulic unit is stopped, the oil feed to the cylinder 7 is stopped, the lock piece 6 is separated from the drum 2 by the spring pressure 9, and the lock is released. The drum 2 can be surely locked or unlocked, and workability is improved.