PURPOSE: To make an output rotary unit high-low speed two-step selectable and forward-reverse rotatable by providing such constitution that a rotary plate contact mechanism and an outer shell internal gear stop mechanism are mutual compensation-actuated with the output rotary unit operated in high speed rotation at the time of actuating the rotary plate contact mechanism and in low speed rotation at the time of actuating the outer shell internal gear stop mechanism.

CONSTITUTION: In the case of rotating an output rotary unit 6 at a low speed, a solenoid 50 is not energized so as to mesh a stop piece 53 with the external tooth 30 of an outer shell internal gear 29, and an electromagnet 44 is not energized to isolate a high speed rotary plate 8 from the output rotary unit 6. In this way, the output rotary unit 6 is rotated at a low speed. In the case of rotating the output rotary unit 6 at a high speed, the solenoid 50 is energized to detach the stop piece 53 from the external tooth 30, the outer shell internal gear 29 of the second planet gear unit 5 is freely rotated, the electromagnet 44 is energized, the high speed rotary plate 8 is brought into contact with the output rotary unit 6, which is rotated integrally with the high speed rotary plate 8. When the high speed rotary plate 8 is brought into contact with the output rotary unit 6, the outer shell internal gear 29 must be freely rotatable, and an outer shell internal gear stop mechanism and a rotary plate contact mechanism are synchronized with each other and mutual compensation-actuated.