Provided is a switching power supply device having a very small switching loss, which includes a rectifier circuit for rectifying a commercial AC voltage, a full-bridge circuit having first to fourth switching element, a transformer having a single primary coil and an N number of secondary coils, an N number of rectifying and smoothing circuits, an output detecting circuit for detecting at least one of voltage and current output from each rectifying and smoothing circuit, and a control circuit. Each rectifying and smoothing circuit includes a rectifying unit, a secondary switching element for controlling an output of the rectified voltage, and a smoothing unit for smoothing the rectified voltage, and the control circuit turns on each secondary switching element during a predetermined time.

A light source drive device includes a supply unit that supplies a high-frequency first drive signal and a second drive signal whose frequency is lower than a frequency of the first drive signal to a discharge lamp which has a pair of electrodes, and a control unit that controls the supply unit so that the first drive signal and the second drive signal of fixed durations are alternately supplied and that controls the supply unit so that the first drive signal is supplied in a case where a voltage between the pair of electrodes is below a threshold.