A switching power supply device includes a switching output circuit, an error amplifier, a slope voltage generating circuit, a PWM comparator, a logic circuit, a switch driving circuit, and a reverse current detection circuit. The slope voltage generating circuit increases a slope voltage from a reset level with a gradient corresponding to an input voltage during an on period of the output transistor, and maintains the slope voltage at an offset level corresponding not to the reset level but to an output voltage during an off period of the output transistor.