An alternating current power supply device is provided with switch elements Q1, Q2 for converting direct current power from a direct current power supply Vin into alternating current power; a transformer T1 for converting the voltage of the alternating current power converted by the switch elements into other voltage; a load 20 connected to an output terminal of the transformer; a first detecting circuit 30 for detecting a first electric signal indicating power to be supplied to the load; a second detecting circuit 40 for detecting a second electric signal indicating the voltage of the direct current power supply; a feedback circuit 50 for generating a feedback signal based on the first electric signal detected by the first detecting circuit and the second electric signal detected by the second detecting circuit; and a control circuit 10 for generating a control signal based on the feedback signal from the feedback circuit and controlling on/off of the switch elements by the control signal so that power to be supplied to the load is at a prescribed value.