PURPOSE: To optimally set the speed reduction starting time of a motor without controlling position setting of a sensor, and to always aim at the stabilized screw fastening by starting speed reduction of a motor after the time equivalent to the delay angle is elapsed from the time when the detecting signal is inputted from the sensor.

CONSTITUTION: In a screw fastening parameter setting unit 12, setting condition written in a memory 14 such as the fastening torque per each screw fastening position, rotating speed and rotating angle of a bit 2, are read out, and the necessary setting signal is given to a motor control unit 11 to turn a motor 1 at a set high rotating speed. On the basis of the detecting signal of a sensor 8 for detecting the time when a screw 3 achieves a position just before sitting, the pulse signal from an encoder 13 is counted, namely, a rotating angle of a bit 2 is measured, and the measured angle is compared with the set angle. At the time when the measured angle exceeds the set angle, rotating speed of the motor 1 is reduced, and thereafter, when reaching to a current value corresponding to the set fastening torque is judged, the motor 1 is stopped.