A switching power supply includes: a switching output circuit generating an output voltage from an input voltage according to ON/OFF control of an output transistor by a pulse width modulation (PWM) signal; a first voltage generating circuit generating a first voltage based on a difference between the output voltage and a predetermined reference voltage; a second voltage generating circuit generating a triangular second voltage; a comparing circuit generating a comparison signal by comparing the first voltage and the second voltage; a clock oscillating circuit generating a clock signal; and a logic circuit generating a PWM signal in response to the clock signal and the comparison signal, wherein the second voltage generating circuit generates the second voltage by adding a first slope voltage having a first slope and a second slope voltage having a second slope according to current flown through the switching output circuit.