PURPOSE: To suppress the shock torque at a small value by providing a control means to rotate a motor at a specific low speed until a screw is fastened perfectly from the condition immediately before it is fastened perfectly.

CONSTITUTION: A condition immediately before a screw is seated is recognized by a learning. Then, it is detected by a detecting circuit that the screw is seated. When the screw becomes to the condition immediately before the seating, the rotation of a motor is stopped by braking by a control device. After that, the motor is restarted, and in this case, the speed of the motor is made at the lowest speed, and the lowest speed is maintained until the screw is seated. And when the screw is seated, the screw fastening is carried out gradually, and the screw fastening is completed at the time when the motor current becomes to a specific current value.