PURPOSE: To provide an automatic driver comprising a thrust load varying mechanism capable of suitably and constantly applying a thrust load to a screw.

CONSTITUTION: A thrust load varying mechanism is constituted by two springs consisting of a relatively weak first spring 13 and a relatively strong second spring 16 for applying a suitable thrust load to a screw N; and a sensor for switching and operating the two springs in early and last stages of screwing, and the sensor detects the movement of a bit attaching member 12 so as to switch and operate the two springs.