Motor encoder

An encoder mounted on a motor axle of an AC servomotor has a single bipolarly magnetized rotational disk, magnetic sensors for outputting X- and Y-signals that differ in phase by 90° in association with the rotation thereof, and a signal processing unit. The signal processing unit computes at predetermined cycles a rotor angle thetaa of a motor rotor on the basis of the X- and Y-phase signals, generates three-phase magnetic pole signals U, V, and W that differ in phase by 120° on the basis of the number M of magnetic poles of a motor, a magnetic pole reference angle thetao, and the rotor angle thetaa; and generates A- and B-phase signals with predetermined pulse cycles that differ in phase by 90° on the basis of the rotor angle thetaa. An encoder that is advantageous for reducing the size and weight of an AC servomotor can be realized because it is sufficient to provide a single assembly composed of a rotational disk and magnetic sensors.