Insulation resistance detection system, insulation resistance detection apparatus and insulation resistance detection method

xAn insulation resistance determining system comprises an insulation resistance determining portion configured by connecting a coupling capacitor, a detection resistor, and a pulse oscillating circuit in series with a common mode potential measuring point, and an insulation resistance determining portion having a filter which processes an AC signal (v) at a junction between the coupling capacitor and the detection resistor, an amplifier, a wave-peak value detection circuit, and a controller. The controller has functions for detecting a drop in insulation resistance in response to an output of the wave-peak value detection circuit; for requesting an HVCPU to change the common mode potential so as to determine a failure of the insulation resistance determining portion; and for determining a failure of the insulation resistance determining portion from an output change of the wave-peak value determining circuit between before and after the common mode potential change.