To electrically reduce the distortion of sine waves to improve the position accuracy by taking sine waves each having a quadrature phase shift from coils mounted at mutually shifted positions and making the phase adjustable.

Coils are formed on both salient poles of a rotor. A sine wave with a phase shift of about 90° is taken from each coil, an a-c reference signal at specified frequency e.g. about 10 kHz is fed to the coils from an a-c signal source 22, differential units 23, 24 demodulate the outputs of the coils into sine waves with phases A, B, LPFs 25, 26 remove noises therefrom, phase adjuster circuits 27, 28 adjust the phases, and gain adjuster circuits 29, 30 adjust the gains. The two sine waves are fed to a decoder 31 to obtain a position detection information. The phase adjustment corrects the accuracy dispersion over the teeth of the salient poles and the gain adjustment absorbs errors resulting from the winding method of each coil, variations of the salient pole cores and circuits, etc.