A switching power supply comprises a switching device, a smoothing circuit, and a control circuit including a current detector for detecting a current flowing through an inductor in the smoothing circuit. The detected current is multiplied by a current control gain to generate a signal PIS. The rectangular wave signal is add to and the signal PIS is subtracted from a voltage detection signal corresponding to the output voltage of the switching power supply. The current control gain is adjusted so that .DELTA.PIS/.DELTA.D becomes closer to a predetermined constant, where .DELTA.PIS represents an amount of change in the signal PIS, and .DELTA.D an amount of change in the duty ratio of a driving signal for the switching device.