A change in torque produced by a motor having two rotors is reduced when the motor operates in a low-velocity and low-torque operational state. The motor has a first rotor and a second rotor, each of which has a permanent magnet, and an output shaft capable of rotating integrally with the first rotor. The second rotor can relatively rotate with respect to the first rotor by a driving force from a phase difference changing driving device, and the phase difference between the rotors (rotor phase difference) can be changed by the relative rotation of the second rotor with respect to the first rotor. When the values of the torque and the rotational velocity of the motor lie within a predetermined region in the proximity of 0, the phase difference changing driving device adjusts the rotor phase difference to a predetermined phase difference for which the strength of a composite field of the permanent magnets is lower than a maximum strength.