A switching mode power supply apparatus includes a conversion unit to convert input power into output power having a predetermined voltage by performing a switching operation, a light emitting unit to emit light if the voltage of the output power exceeds a predetermined threshold voltage, a light receiving unit to receive the light emitted from the light emitting unit and output a signal indicative of the voltage of the output power, a switching controller to control the switching operation of the conversion unit according to the voltage of the output power indicated by the signal output from the light receiving unit, and a disconnection unit to disconnect power applied to the light receiving unit if a voltage of the power applied to the light receiving unit exceeds a predetermined trigger voltage.