To provide a pulse width modulation circuit in which the output frequency or the minimum pulse width is kept constant even at the time of pulse width modulation.

The pulse width modulation circuit comprises means for generating a pulse depending on the ratio of first and second currents, a current source means generating a current equal to the sum of the first and second currents, a modulation signal generating means for modulating the duty ratio of a pulse by generating a modulation signal and varying the ratio of first and second currents, and a power supply control means for controlling the output frequency by controlling the current of the current source means.