PURPOSE: To control the fastening force by fastening or loosening a bolt joint without damaging a seat face and a screw section.

CONSTITUTION: A piece 10 is fixed at the end section of a bolt fastening a member together with a nut, and a female screw section 32 couplable with a male screw section 12 formed on the peripheral face of the piece 10 is formed on the inner peripheral face of a rotor 30. A circular pedestal 20 receiving the rotation of the rotor 30 is arranged on the seat face surrounding the nut, and the rotor 30 is provided with a coupling section 34 coupling the rotary shaft of a driving tool to rotate the rotor 30 and an opening 35 operating an apparatus fastening the nut.