To accurately grasp an internal condition of a charge accumulating device, and to precisely find a power quantity all the time even under various conditions in practical use.

The power quantity P is found by composition of a power quantity PV based on a release voltage and a power quantity PC based on a current weighted with a weight w changed from time to time in response to a using condition of a battery. A computed value is thereby precluded from being fluctuated by fluctuation of a load under the practical using condition, and precision of the computed value is thereby precluded from getting low by fluctuation of a sampling data, so as to obtain the power quantity reflected accurately with the internal condition of the battery.