The car power source apparatus is provided with a leakage detection circuit 3 having a battery 1 with a plurality of battery units 2 connected in series, a first series circuit 11 made up of first leakage detection resistors 12 and a first leakage detection switch 13 to connect the first connection node 10 of the series connected battery units 2 to ground 9, a second series circuit 21 made up of second leakage detection resistors 22 and a second leakage detection switch 23 to connect the second connection node 20 of the series connected battery units 2 to ground 9, and voltage detection circuits 4 to detect voltage of the first leakage detection resistors 12 and the second leakage detection resistors 22. Further, the car power source apparatus is provided with a failure detection circuit 5 to control the first leakage detection switch 13 and the second leakage detection switch 23 ON and OFF and determine failure of the leakage detection circuit 3 from voltages detected by the voltage detection circuits 4.