A leakage detector which can detect leakage correctly in both the DC high voltage circuit and the AC high voltage circuit of a vehicle-mounted power supply system. Under a state where a contactor (17) is turned off, all IGBT elements (switching elements) (76) of an IGBT inverter circuit (12) are turned on and an AC signal VS is applied to an application point P. Leakage is detected by comparing the measured voltage at a voltage measuring point Q with a threshold.