The transient response of an output voltage to a load fluctuation is improved, in a switching power source that carries out a PWM control. In a DC-DC converter wherein a switching element of an output stage is controlled by a drive signal, whose pulse width is set at a minimum value, output from a PWM signal generating circuit based on an output voltage output from an error amplifier in accordance with the difference between a feedback voltage in accordance with an output voltage of the output stage and a reference voltage, there is provided a minimum pulse width detector circuit that supplies a current to a phase compensation capacitor when the pulse width of the drive signal is at the minimum value, thus preventing the output voltage from dropping below a value corresponding to the minimum value when the load fluctuates, and improving transient response characteristics of the output voltage.