PURPOSE: To realize sure screw tightening action regardless of the posture of the other member and the shift of screw holes, in the case of an automatic screw tightening device equipped with a screwing mechanism at the arm tip portion of a multijoint robot.

CONSTITUTION: A force sensor 71 to detect force that is received at the time of a screwing mechanism 2 conducting screwing, is fitted to the base end portion of the screwing mechanism 2, and the output end of the force sensor 71 is coupled to a robot control circuit 15. The robot control circuit 15 sets, at the time of screwing, the target posture of the bit 23 of the screwing mechanism 2 on the same line with force vector, and conducts the feeback control of the posture of a robot arm.