PROBLEM TO BE SOLVED: To detect a speed frequency signal in high precision without particularly providing a frequency generator, control rotating speed of a motor in high precision, and make vibration and noise small. SOLUTION: A counter-electromotive force detection circuit 1 outputs a formed signal m and a speed frequency signal FG by dividing a comparison signal u of a stator winding 11 by detecting zero-crossing points of the counter electromotive force induced in each phase of the stator windings 11, 12, 13. A position signal generator circuit 2 outputs position signals d-i of 6 phases in trapezoid-shaped waveform corresponding to the formed signal m. A circuit 3 compounds trapezoid-shaped waveform position signals d"-i" of 6 phases of which the base width of the stator winding 11 is narrower than those of the stator windings 12, 13, and supplies these signals to a power supply circuit 4. The circuit 4 provides a rest period of current zero with making an electrification width less than 180 degrees of a current which flows through the stator winding 11 corresponding to the position signals d"-i", supplies the stator windings 12, 13 trapezoid-shaped current to make the electrification width of the current 180 degrees, and detects the speed frequency signal FG in a high precision at the counter electromotive force detection circuit 1.
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