A DC power supply circuit outputs an output voltage, which varies with an input voltage. The output voltage therefore occasionally deviates from a normal output voltage range although the DC power supply circuit is normal. However, the DC power supply circuit converts the input voltage in a predetermined relation between the input voltage and the output voltage. When the output voltage of the DC power supply circuit deviates from a predetermined normal output voltage range, a determination circuit determines that the DC power supply circuit is abnormal when the input voltage and the output voltage of the DC power supply circuit are not in the predetermined relation. It is thus prevented that the DC power supply circuit is erroneously determined to be abnormal although operating normally. As a result, it is possible to eliminate erroneous determination in determining the operation state itself.