PURPOSE: To mount/dismount nut and washer smoothly by employing an air cylinder mechanism in a stud bolt mounting/dismounting device for securing an upper mirror to a pressure container of boiling water nuclear reactor thereby relieving impact onto the nut.

CONSTITUTION: A pressure feeding unit 38 is provided as a means for moving a member 8 which rotates and grips a large nut 4 and an associated washer 5 in axial direction, and the pressure to be fed from the unit 38 is made switchable between high and low levels. Consequently, load in axial direction is regulated when the nut 4 and the washer 5 are mounted or dismounted so as to prevent over loading. The nut 4 and the washer 5 are gripped or removed in soft touch by feeding pressure for cancelling the weight of the nut 4 and the washer 5. Since a movable shaft 19B moves up and down when the supply pressure is switched from high level to low level or low level to high level, the device can be set to predetermined condition in short time.