A soft start circuit is connected to a pulse width modulation controller including an oscillator, and a functionality of modulating amplitude to a pulse width and a power supply includes the soft start circuit. The soft start circuit includes a frequency controlling unit, a duty ratio establishing unit, and a variable switching unit. The frequency controlling unit generates first and second parameter signals for determining a frequency signal frequency by a power source from the PWM controller and provides them to the PWM controller. The duty ratio establishing unit generates a third parameter for determining amplitude of the frequency signal generated by the PWM controller according to a reference voltage, and provides it to the PWM controller. The variable switching unit determines whether it is a first predetermined time from a start-up state, and controls the first parameter of the frequency controller during the first predetermined time.