A switching power supply device includes a slope compensation circuit configured to start slope compensation for suppressing subharmonic oscillation in accordance with a timing signal from an oscillation circuit. The oscillation circuit is provided with a first circuit, which generates a signal of a fundamental oscillation frequency, and a second circuit, which applies logic processing to the signal of the fundamental oscillation frequency to form the timing signal. Thus, it is possible to provide a switching power supply device in which a variation in a start timing of slope compensation can be suppressed.