PURPOSE: To avoid occurrence of an action delay and an error output by selecting the specific one of those pulses produced by combinations of plural bits of the input data and securing an OR of the selected pulse.

CONSTITUTION: This modulation circuit consists of a PWM (pulse width modulation) basic pulse generating circuit 6 which produces circularly and repetitively the pulses of different pulse widths, a combination selecting circuit 7 which selects the specific one of those pulses produced by the circuit 6 according to the ON/OFF combinations of plural bits forming the input data to perform the PWM, and an output means which secures an OR of the specific pulse selected by the means 7 and outputs it as a PWM output. Thus an error output is never produced despite a coincidence unable state since no coincidence detecting circuit is used. In addition, no action delay occurs owing to absence of a latch circuit. Then a PWM output is quickly obtained.