PURPOSE: To detect the speed of a motor highly accurately by detecting the time required for the displacement of a specific position of a rotor magnet between two Hall elements.

CONSTITUTION: When current is supplied to driving coils 3aW3d, a six-pole magnetized rotor magnet 1 is rotated. Consequently voltage with the phase difference of 90° electric angle is taken out by a Hall element 41 opposed to the center of the N pole of the magnet 1 at the interval of 90° electric angle and a Hall element 42 opposed to the part between the N and S poles. Since "0" intersecting point of the waveform of the voltage corresponds to the time a specific position B between the S and N poles of the magnet 1 passed through the elements 41, 42, the motor speed is detected from the time difference of said passing time. Said magnetized status makes it possible to omit a frequency generator or the like of which accuracy is sharply changed and the speed of the Hall motor can be detected highly accurately.