An adapter power supply of the invention comprises a switching element that performs switching of an input voltage obtained by rectifying an input AC voltage to deliver switched voltage to a primary winding of an isolating transformer, a diode that rectifies a voltage obtained across a secondary winding of the isolating transformer to obtain a DC output voltage, and a thermoelectric conversion element that performs thermoelectric conversion to variably set the DC output voltage corresponding to a temperature difference between a heated temperature of the switching element or the diode and an atmospheric temperature. The thermoelectric conversion element is disposed between a heat sink attached to the switching element or the diode and a package for containing a main body of the adapter power supply.