A power supply apparatus includes a power supply part including a plurality of power supply units, a first comparison unit comparing a first voltage of the power supply part with a first reference value, a directing unit issuing a voltage fall direction to one of the power supply units so as to lower a voltage setting value when the first voltage is higher than the first reference value, a second comparison unit comparing a second voltage of the power supply unit obtained after the voltage fall direction is issued with the second reference voltage value, and a determining unit determining that the power supply unit to which the voltage fall direction issued corresponds to a failure unit, when the second comparison unit detects that the second voltage obtained after the voltage fall direction is issued fell in the voltage level lower than the second reference voltage.