A switching power supply device has a drive circuit that can minimize a loss while being compact. The drive circuit turns on and off a high-side switching element (MOSFET) according to a positive or negative voltage developed at a tertiary winding of a transformer. The drive circuit includes a control unit that detects the development time, during which the negative voltage is developed at the tertiary winding, as the on time of a low-side switching element, and makes the on time of the high-side switching element nearly or substantially equal to the development time.