A voltage measuring circuit 11 measures a voltage of a block B1. Then, a voltage measuring circuit 12 measures a voltage of a block B2. Then, the voltage measuring circuit 11 measures again the voltage of a block B1. A low voltage CPU judges that an in-vehicle battery is abnormal if the measured voltage is out of predetermined upper and lower limits, or a voltage difference between the block B2 and the block B1 measured again is more than a predetermined value.