To achieve power saving by reducing power consumption when in a no-load power supply state in an AC adaptor having a switching power supply configuration.

When in a no-load power supply state, the detection result of a voltage detecting part 22y is corrected by a detection/correction part 27. On the basis of the correction, a switching control part 13 controls a switching part 11 such that the DC power supply for power feeding of an output processing part 18 has a lower voltage than a voltage before the correction. Output of a fourth winding 10d formed by additional winding of a winding wire to a third winding of a transformer 10y is rectified and smoothed by an auxiliary power supply part 37 so as to form the DC auxiliary power supply that has a voltage equal to or over a minimum operating voltage of the switching control part 13. Instead of the control power supply of a control power supply part 26, the auxiliary power supply of the auxiliary power supply part 37 is supplied to a power-supply terminal Vdd of the switching control part 13 so as to reduce power consumption by preventing the switching control part 13 from being restarted due to injection of the DC power supply on the input side of an input processing part 7 in a no-load power supply state.