PURPOSE: To materialize stable motor drive, by getting pulses at every 120° based on the terminal voltage of a motor and by outputting driving waveforms with phases corrected as a result of the measurement of this pulse interval.

CONSTITUTION: The terminal voltage of a brushless motor is supplied from input terminals 1a∼1c to non-reverse input terminals of comparators 4a∼4c through filters 20a∼20c. The non-reverse input terminals of the comparators 4a∼4c are earthed. The output of comparators 4a∼4c is sent out to a differential pulse generation circuit 21 and to a microcomputer 22. The output of the differential pulse generation circuit 21 is sent out to the microcomputer 22, which outputs driving waveforms of an inverter circuit 17 correcting phases as a result of the measurement of pulse interval from the differential pulse generation circuit 21.