PURPOSE: To enable a large size bolt to be tightened by a constitution in which the counterforce of a torque wrench is supported by an object to be screwed so as not to be transmitted to the arm.

CONSTITUTION: A torque wrench 1 is fixed on a torque wrench attaching plate 3 by means of bolts 11. The attaching plate 3 is mounted such that it rotates freely around the axis of the socket 2 of the torque wrench, with respect to the base plate 4 fixed to the end of a robot arm 9 by means of bolts 12. A return spring 6 is further provided between the torque wrench attaching plate 3 and the base plate 4 to make the attaching plate 3 return to its original position after it is rotated by the tightening counterforce. Thereby, a large size bolt and nut can be tightened with a robot arm having relatively low rigidity and with a motor having relatively small output of driving the arm.