An insulation resistance determining system (50) comprises an insulation resistance determining portion configured by connecting a coupling capacitor (52), a detection resistor (54), and a pulse oscillating circuit (56) in series with a common mode potential measuring point (30), and an insulation resistance determining portion having a filter (58) which processes an AC signal (vx) at a junction between the coupling capacitor (52) and the detection resistor (54), an amplifier (60), a wave-peak value detection circuit (62), and a controller (70). The controller (70) has functions for detecting a drop in insulation resistance in response to an output of the wave-peak value detection circuit (62); for requesting an HVCPU (40) 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 (62) between before and after the common mode potential change.