A switching power supply includes a first switching element connected between a primary winding of a transformer for the switching power supply and the ground, a shunt regulator serving as an output voltage detection circuit configured to detect an output voltage on a secondary winding side of the transformer, a photocoupler configured to transmit the output voltage to a control circuit, a second switching element configured to receive a stop signal for stopping operation of the switching power supply, and a photocoupler drive circuit configured such that when the second switching element has received the stop signal, power output from a low-frequency transformer power supply or the switching power supply is supplied to the photocoupler, and the stop signal is transmitted to the control circuit via the photocoupler.