is larger than the value rlx for example equals to rla if the detection accuracy is in a state that the value of the resistor rl is detected slightly larger than the actual value the value of the insulation resistor is detected slightly larger than the actual value due to the dc bias characteristics of a ceramic capacitor when the ceramic capacitor is used as the flying capacitor thus such a determination is not made that the occurrence of the insulation deterioration is detected despite that the insulation deterioration does not occur yet in contrast when the value of the resistor rl is larger than the value rlx for example equals to rlb the value of the insulation resistor is detected slightly smaller than the actual value due to the dc bias characteristics of the ceramic capacitor thus such a determination is not made that the value of the grounding resistor does not reach the value rlx and so the insulation state is good despite that the insulation deterioration has occurred thus the insulation measurement circuit 10 according to the embodiment is arranged to remove the influence of the dc bias characteristics on the capacitor c using the ceramic capacitor which is the bottle neck for realizing the high detection accuracy when the value of the resistor rl is near the value rlx to this end although the detailed explanation will be made later a voltage vc 1 p or a voltage vc 1 n set to the capacitor c is made coincide with a voltage vo set to the capacitor c by the power supply v and also the charge time periods at this.