The power source apparatus has high-voltage-side power supply circuitry 4 that supplies driving battery 1 power to the first measurement circuit 2 and the second measurement circuit 3, isolation circuitry 5 that isolates output from the first measurement circuit 2 and the second measurement circuit 3 and outputs it to the vehicle-side, and low-voltage-side power supply circuitry 6 that supplies vehicle auxiliary battery 7 (12V) power to the isolation circuitry 5. The first measurement circuit 2, which is powered by the high-voltage-side power supply circuitry 4, outputs voltage signals to the vehicle-side via the first isolation circuit 5A, which is powered by the first low-voltage-side power supply circuit 6A, and the second measurement circuit 3, which is powered by the high-voltage-side power supply circuitry 4, outputs battery 10 error signals to the vehicle-side via the second isolation circuit 5B, which is powered by the second low-voltage-side power supply circuit 6B.