A switching power supply includes a series resonant circuit that includes a resonant inductor and a resonant capacitor connected in series with a primary winding of a converter transformer. By controlling turning on and off of first and second switching elements in a complementary manner, current is supplied to the series resonant circuit. A third switching element connected on the secondary side of the converter transformer is synchronized with the first switching element, and a fourth switching element is synchronized with the second switching element. If a switching frequency is less than a resonant frequency, turning on of the third and fourth switching elements is synchronized with turning on of the first and second switching elements, and turning off of the third and fourth switching elements is controlled, without being synchronized with turning off of the first and second switching elements, after half a resonant period has elapsed.