In a pulse width modulation circuit, which generates an output whose pulse width is modulated by controlling a duty ratio of an oscillation circuit adapted to generate an oscillation at a frequency determined by a resistor and a capacitor to be electrically charged via the resistor, there is provided a resistance value varying means to vary a value of the resistor. The value of the resistor is varied by shorting the both ends of the resistor with a transistor, whereby the capacitor is charged in a reduced amount of time realizing a high-speed pulse width modulation circuit.