A car power source apparatus is provided to include a driving battery 11 outputting an output voltage for driving a car, a monitoring and control circuit 21 monitoring and controlling the driving battery 11 and connected to an auxiliary battery 3 via power supply lines 2, and a battery case 1 housing the driving battery 11 and the monitoring and control circuit 21 and connected to a car chassis ground 5 via a ground line 4. Further, the power source apparatus is provided with a connection detection circuit 40. This connection detection circuit 40 determines the condition of the connection of the battery case 1 and an auxiliary battery 3 power supply line 2 via the ground line 4 and the car chassis ground 5 to determine the condition of the ground line 4 connection.