A speed changing mechanism for use with an electric rotary tool, an electric press-attaching tool, an automatic screw tightening tool or the like, includes planet gears, a planet gear support plate integrally formed with an internal gear and provided with an external gear formed outwardly of the internal gear. The planet gears are rotatably supported by a second planet gear support plate mounted on a driven shaft and formed with a second external gear. The planet gears are engaged with the interanl gear and a sun gear provided on a one-way clutch. A cylindrical member having an internal gear is slidably fitted with at least one of the first and second external gears. The speed changing mechanism can be set to have a ratio of the rapid rotation speed to the slow rotation speed of 2.5/1 or less.