The cost of hardware and the cost of calculation are reduced in the case where a plurality of assembled batteries are used. The amount of change in the voltage of each battery constituting an assembled battery is sequentially measured with a voltage monitor and abnormality is detected from the correlation between the amounts of changes in the voltages of batteries at the same period. Furthermore, abnormality is detected on the basis of the amounts of changes up to the previous step. Inference is made by a reference parameter acquisition system using an IIR filter or an FIR filter.