A threaded member tightening tool for setting the length of drive time to elapse before rotation is switched from high-speed rotation to low-speed rotation. A motor-driven screwdriver system in which a screwdriver bit is first driven at a predetermined high rotational speed only for a high-speed drive time and thereafter driven at a predetermined low rotational speed lower than the high rotational speed. A control unit drive-controls an electric motor so that the screwdriver bit is rotationally driven at a setting rotational speed, measures a tightening time required until a Hall element detects that a threaded member engaged with the screwdriver bit has been tightened completely, calculates an initial setting time by multiplying the tightening time by a predetermined positive value less than 1 and a value obtained by dividing the setting rotational speed by the high rotational speed, and sets the high-speed drive time to the initial setting time.