To provide an actuator 20 for selectively displacing an output member 23 against the opposed load L, in response to an output 24 of a motor 23.

This actuator includes a first power transmission mechanism 50 for displacing the output member to the motor with a first nominal ratio to the motor output, a second power transmission mechanism 60 for displacing the output member to the motor with a second nominal ratio to the motor output, and selectors 34, 39 connecting the motor output to the output member, only via the first power transmission mechanism 50 when the load is less than a predetermined value, and connecting the motor output the output member via the first and second power transmission mechanisms 50, 60 when the load is more than the predetermined value. The power applied to the load by the output member is the sum of the power transmitted via the first and second power transmission mechanisms.