In a power supply device according to one embodiment, a reference signal (Vref1), which changes from a value corresponding to a maximum current in a full lighting state to that corresponding to a minimum current in case of a deepest dimming depth, and a reference signal (Vref2), which changes from a value corresponding to a load voltage at the time of a maximum current in a full lighting state to that corresponding to a minimum current in case of the deepest dimming depth, are prepared in accordance with dimming depths of a dimming signal. In a shallow dimming depth region close to a full lighting state, the reference signal (Vref1) is selected to apply constant-current control to light-emitting diodes in a current control mode. In a deep dimming depth region, the reference signal (Vref2) is selected to apply constant-voltage control to the diodes in a voltage control mode.