Switching power supply

In the switching power supply, a load state judgment circuit (25) judges the state of a load (20) according to a pulse signal VG to be used for ON/OFF control of a MOS transistor (8) as a switching element in a DC-DC converter (127). In order to reduce a power consumption of the switching power supply, a PFC ON/OFF switching circuit (24) stops the operation of the power factor improving converter (126) when the judgment result indicates a light load state, and starts the operation of the power factor improving converter (126) when the judgment result indicates a heavy load state.