PURPOSE: To accurately set the seating point, prevent excessive tightening and greatly improve the tightening accuracy by applying the braking torque as it cancells the inertia force of a nut runner at the point of seating and the completion of the tightening.

CONSTITUTION: With a joint part 5 caught by a tightening attachment 3 stuck to the end of the turning shaft of a motor 1, the attachment 3 is turned by the driving force of a motor 1 and then stopped. At this time the braking torque calculated from the expression of T=Id2θ/dt2 (where T standing for braking torque, I for inertia moment of the turning shaft of the motor and θ for the angle of turning) is applied to said shaft in the direction reverse to the turning moment simultaneously with the stop point of current supply to the motor 1 so that the turning shaft 1 of the motor 1 is stopped instantly. As a result, at the time of seating or completion of the seating of the nut runner 9, the turning action thereof is stopped simultaneously when the current supply to the motor is stopped, whereby preventing the excessive tightening.