To provide a low-cost, thin actuator by remarkably shortening the length in the axial direction of the actuator and by decreasing the number of components that constitute a detector.

This actuator comprises a rotor 4 that has a plurality of motor magnets 6 fixed to a rotating shaft 12; a stator 1 that has armature windings 3 that cause rotating magnetic field to a stator core 2 that is made to face the outer peripheral side of the rotor 4 via a gap; a rotating portion of a rotational position detector fixed to the rotating shaft 12; and a fixed portion of the detector facing the rotating portion of the detector. The rotor 4 constitutes a cup-shaped rotor yoke 15 that is provided integrally with the rotor in such a way that the rotating shaft 12 is passed through the center of the rotor yoke. Also, the motor magnets 6 are attached fixedly to the outside circumference of the rotor yoke 15. An encoder magnet 7 that becomes the rotating portion and a magnetic detector 8 that becomes the fixed portion of a magnetic encoder making up of the rotational position detector are arranged to face each other at the inner peripheral side of the rotor yoke 15.