To provide an automatic screw fastening machine that achieves screw fastening, performed by high-speed low-torque drive and low-speed high-torque drive, by one motor and fastens a screw at high speed and with high accuracy.

The automatic screw fastening machine includes: a high-speed low-torque transmission system directly connected to a rotary drive source 3 to transmit high-speed low-torque drive to an output shaft 9; a low-speed high-torque transmission system connected to the rotary drive source 3 via a deceleration means 11 to transmit low-speed high-torque drive to the output shaft 9; clutch means 7, 11 that switch the transmission systems according to a rotational load, acting on the output shaft 9, such that the rotary drive is transmitted to the output shaft 9 by the high-speed low-torque transmission system in a temporary fastening step while the rotary drive is transmitted to the output shaft 9 by the low-speed high-torque transmission system in an actual fastening step; and a control unit that controls such that the rotary drive source 3 is subjected to high-speed rotation in the temporary fastening step while the rotary drive source 3 is subjected to lower-speed rotation than that of in the temporary fastening step and to higher-speed rotation than that of in the post-stage of the actual fastening step in the pre-stage of the actual fastening step.