To provide a switching power supply where energy saving of an electrical apparatus can be realized, while the risk of breakdown of switching elements is excluded, and its control method.

When an electrical apparatus body 1 enters energy-saving mode and the signal from the body 1 is received, oscillation of a switching pulse width control IC1 which controls the output voltage of a switching power supply is stopped. When low-frequency pulses from the body 1 are received, control is performed so that the switching operation is performed by using the low frequency pulses.