The present invention is an in-vehicle charger for detecting ground faults originating in sections in which alternating current is flowing. This device is an in-vehicle charger (100) for charging a vehicle-mounted battery, wherein the device is provided with: a bridge rectifier (14) for converting alternating current supplied from a power source to direct current; a ground fault detecting circuit (21) for outputting a test voltage when a test current flows in a circuit in the in-vehicle charger (100) and, based on changes in the test current in response to the presence or absence of a ground fault resistor, for detecting a ground fault in the circuit of the in-vehicle charger (100); and a controller (23) for controlling the ground fault sensing circuit (21) so as to output a test voltage exceeding the forward voltage of a diode provided by the bridge rectifier (14).