<p>A reference range of tightening torques is predetermined relative to the rotation angle of a threaded member as tightened up normally. In tightening up a nut on an externally threaded member, the tightening torque is detected by a torque sensor every time the nut is rotated through a specified angle, and whether the tightening torque relative to the rotation angle is within the reference range is judged by a microcomputer. The result of judgment is indicated by a light-emitting diode and a buzzer. <IMAGE></p>
