A switching power supply includes a control circuit controlling ON and OFF of switching devices Q1 and Q2 and having an error amplifying circuit that controls a DC output voltage at a constant preset value, an oscillator circuit that controls the switching frequency in response to the output signal level FB of an error amplifying circuit, and a pulse width control circuit PWM that controls the pulse width in response to the output signal such that the ON-periods of switching devices Q1 and Q2 are equal to each other. The ON and OFF of switching devices Q1 and Q2 is controlled based on the output from oscillator circuit VCO when the output signal is higher than a threshold level. The switching frequency is fixed when the output signal level FB is lower than the threshold level.