PURPOSE: To shorten a time required for transfer to a subsequent fastening work after completion of fastening of a bolt and a nut and to improve working efficiency by providing a switching valve unit by which an oil pressure circuit to a pilot valve is switched to the backward movement direction of a drive rod when a working oil pressure is reduced.

CONSTITUTION: Operation of a hydraulic pump is stopped by means of the spool 6d of a pilot valve interlocked with the drive rod 5a of a hydraulic cylinder 5, and a switching valve unit 7. When a working oil pressure is reduced, working oil flowing from the switch valve unit 7 toward the pilot valve flows in a direction in which the drive rod 5a is moved backward. Thus, when a frame is brought into contact with a reaction force support after fastening of a bolt and nut or completion of fastening, operation by the hydraulic cylinder 5 does not work on the bolt and nut, whereby a constraining force between the reaction force support and the frame is rendered ineffective. As a result, working time can be shortened.