PURPOSE: To improve a fastening workability by weakening the transmission of a rotation torque by a friction clutch means located between a spindle and cam shaft with the load of the cam shaft received from the spindle becoming smaller according to a thread fastening motion being completed.

CONSTITUTION: The transmission of a rotation torque by a friction clutch means 12 between a spindle 10 and cam shaft 4 becomes weak with the load of the cam shaft 4 received from the spindle 10 becoming small according to a thread fastening motion being completed. The engagement of an engaging clutch means 13 is dissociated with the main gear 9 whose rotation from a driving device 2 is continued being positively moved in the dissociative direction in the engaging clutch means 13 by the energizing forces of a cam means and spring 14. Consequently the clutch means can be subjected to dissociation motioning smoothly at the fastening completion time in a motor screw driver, as a result, the generation of an abnormal machine sound is eliminated, and the fastening workability is improved.