To provide a power screw driver formed so that the power ground and electrostatic ground of a motor may be separated from each other.

This power screw driver is provided with a motor 11 connected to a power source by a 3-pin terminal including a ground pin through a noise filter 15, reduction gear mechanism 12 connected to the driving shaft 11a of the motor, a chuck 13 fitted to the output shaft 12a of the reduction gear mechanism, a driver bit 14 fitted to the chuck, whereas the motor and the noise filter are connected to a power ground line G1. In the power screw driver 10, the driver bit 14 and the motor 11 are electrically insulated each other by an insulating means 16.