A power supply apparatus includes: a main switch element connected to a terminal of a lamp and configured to perform a switching operation; an AC to DC converter configured to an AC to DC converter configured to generate DC power by using a signal of an AC hot line and a signal of the terminal of the lamp; a comparator configured to compare a comparison input signal with a predetermined first reference voltage; and a control signal generator configured to generate a control signal for controlling an operation of the main switch element by using an output signal from the comparator.