To stop power supply to a load before an overvoltage occurs even if an AC power source voltage does not match the conversion ratio of input/output voltages of the DC power source circuit, in a power supply device provided with a DC power source circuit that can change over the conversion ratio of the input/output voltage.

The power supply device 11 is provided with a DC power supply circuit 15 that can change the input/output voltage conversion ratio, a triac 14 provided at the AC input side of the DC power supply circuit 15, a voltage detection portion 19 that detects a voltage across terminals of the series circuit of smoothing capacitors 17a and 17b, and a control device 20. This control device 20 compares a detected voltage by the voltage detection portion 19 with a preset reference voltage for judging erroneous setting from the time when the triac 14 is turned on, and turns off the triac 14 when the detected voltage exceeds the reference voltage for judging the erroneous setting.