A power source (120) supplies an input voltage (V 1). A voltage conversion circuit (130) converts the input voltage (V1) from the power source (120) into an operating voltage (V2) to be used to drive an electric load (150). A detection circuit (170) measures an insulation resistance value (R) on the output side of the voltage conversion circuit (130). A control circuit (160) controls a voltage conversion ratio (K (K=V2/V1)) in the voltage conversion circuit (130), which is expressed as a ratio of the operating voltage (V2) to the input voltage (V1), in accordance with the insulation resistance value (R) detected by the detection circuit (170). The control circuit (160) sets the voltage conversion ratio (K) such that the operating voltage (V2) becomes lower at the time of degradation of an insulation resistance than at the time of normal operation thereof.