To achieve co-existence of a stability and a responsiveness upon gear shift by varying the deceleration ratio of rotation transmitted from an electric motor to an output shaft according to a shift range.

A speed reduction ratio variable part 12 is formed of transmission gears 13 and 14. The output shaft 11 is turned by the electric motor 2 through a speed reduction gear mechanism 3, and the shift range of an automatic transmission is switched according to the turning angle of the output shaft. In this case, the speed reduction ratio is increased between P range and R range by meshing the transmission gear 13 with a counter gear 9 to provide a large rotational torque to the output shaft 11. Between R range and L range, the speed reduction ratio is decreased by engaging the transmission gear 14 with a counter gear 5 to increase a responsiveness upon the transmission of rotation. Thus, a switching operation on P range requiring a large drive force can stably be performed, and the switching operation on the other ranges can be performed smoothly.