In an actuator, an electric power supply to a position-detecting detector for an object to be driven can be carried out in a timely manner during a power failure to thereby suppress an electric power consumption in the position-detecting detector, whereby duration time of a power-failure compensation battery can be prolonged, and a variety of detectors can be selected and utilized without limitation in accordance with a use environment and usefulness. During the power failure, a switching means 40 establishes a linking state in which an input shaft 11 of a reduction mechanism 10 and a manual inputting means 30 are linked to each other. When the linking state is detected by a detecting means, an electric power is fed from a battery 80 to a rotation detector by a control section 70, so that the control section 70 can obtain a displacement of the object to be driven, based on a signal output from the rotation detector. In a state in which the input shaft 11 of the reduction mechanism 10 can be manually rotated, the electric power is fed to the rotation detector. Thus, a useless consumption of the electric power of the battery 80 can be suppressed, and it is possible to suitably obtain the displacement of the object to be driven without an interruption even if the power failure happens.