To provide a screw tightening device capable of automatically tightening a screw in the direction inclined to the direction of the screwing axis without any defective engagement.

A drive shaft 15 comprises a large diameter part 15a and a small diameter part 15b arranged on a base end of the large diameter part 15a. The angle of inclination of the drive shaft 15 when a bolt 10 held by a socket 16 is located in a section before the tightening by a tightening part 9 is started is set to be a predetermined value directed to an end of the tightening part 9 by passing through and supporting the drive shaft 15 by a shaft guide 18 having a support hole 18a of substantially same inside diameter as the large diameter part 15a while the angle of inclination of the drive shaft 15 when the bolt 10 held by the socket 16 is located in a section after the tightening is started is set so as to permit a predetermined oscillation. The socket 16 is connected to a tip of the drive shaft 15 in an oscillating manner, and urged in the axial direction by a solenoid coil 17.