PURPOSE: To improve accuracy of detecting a rotating speed by providing means for outputting one pulse at each predetermined rotation of a rotor by using a predetermined logic signal.

CONSTITUTION: Two Hall elements l, 2 are added at an interval of π/n radian of a mechanical angle to a stator of a motor having a rotor including a plurality (n) of poles. Thus, voltages of first and second sine waves having a phase difference of π/2 radian of an electric angle are obtained from the elements 1, 2 upon rotation of a rotor 20b. Further, the first and second sine wave voltages are formed to have equal width, added or subtracted to obtain two sine wave voltages having a phase difference of π/4 radian of an electric angle to the first, second sine wave voltages. A square wave pulse train of π/n of period of mechanical angle is obtained by sequentially using the four sine wave voltages having a phase difference of π/4 radian of an electric angle to reduce a rotary angle error of the rotor 4b per one pulse.