In a power supply device, a primary winding of a transformer is connected in series to a main switching element, and is connected in parallel to a series circuit of a resonance capacitor and a sub switching element. A secondary winding of the transformer is connected in series to a series circuit of a rectifying element and a resonance inductor. The capacitance of the resonance capacitor and the inductance of the resonance inductor are each set such that, when the sub switching element is in an on state, the waveform of the voltage that is generated between the first main terminal and the second main terminal of the main switching element takes a protruding curve shape, the voltage being generated due to a resonance phenomenon of at least the resonance capacitor and the resonance inductor.