The power supply apparatus includes a circuit board on which mounted are a transformer, a switching element connected in series with a primary winding of the transformer, a rectification element connected with a secondary winding of the transformer, a detection unit detecting a temperature of the rectification element when the transformer is supplied with power by a switching operation of the switching element, and a resistance element connected in parallel with the rectification element. The resistance element is mounted at a predetermined mounting section among a plurality of mounting sections on the circuit board.