To save power consumption in the standby status.

This power supply circuit comprises a standby transformer(ST) for connecting one side of AC to the primary side and rectifying the secondary side, a switch element 3 between the other side and ST, an auxiliary power supply part 1 for rectifying the AC between AC and ST 4, an AC input driver 2 for turning on and off the switch element 3 with a prescribed timing, a rectifying circuit in the secondary side of ST 4, a standby power supply 14 for receiving the DC voltage, first and second reference voltage units 10, 11 connected to the common contact of the output side of the rectifying circuit 5 and ST power supply 14 to set the first and second reference voltage from the DC voltage, first and second voltage comparators 8, 9 for inputting the first and second reference voltages and comparing the DC voltage and reference voltage, a switching control section 7 for generating the pulse voltage of the prescribed timing, based on the comparison result and first and second photocoupler switches 6a, 6b that turn on and off the AC input drive unit with the output of the switching control 7.