To obtain a power supply controlling circuit which improves the efficiency of a power supply and causes no noise to a load circuit.

While an electronic circuit, a main load circuit, is operating, a main power supply circuit comprising a transformer 4, a rectifier circuit 5, a capacitor 6 and an automatic voltage controlling circuit 7 is controlled to an operating state, and a switching power supply circuit 9 to a stop state so that the main power supply circuit supplies a control circuit 1, a backed-up circuit with a back-up voltage. While the electronic circuit is not operating, the main power supply circuit is controlled to a stop state, and the switching power supply circuit 9 to an operating state so that the supply of the back-up voltage is changed over from the switching power supply circuit 9. Furthermore, when the back-up voltage to the control circuit 1 is changed over, the back-up voltage is supplied to the backed-up circuit from both the main power supply circuit and switching power supply circuit 9 for a predetermined period of time.