A battery ECU estimates the SOC by integrating the battery current measured by a current sensor, and the battery voltage V.sub.nis measured by a voltage sensor, and the battery temperature T.sub.nis measured by a thermometer if the fluctuation of the charging/discharging current is great. If the number m of estimations of SOC.sub.nis m.ltoreq.10, m is incremented. The battery internal resistance R.sub.nis estimated from the measured battery temperature T.sub.nby using a correlation map showing the correlation between the previously stored battery temperature T and the battery internal resistance R. An estimation charging/discharging current I.sub.nis determined using the measured battery voltage V.sub.n,the battery open voltage V.sub.ocvn-1determined on the basis of the previously estimated charged state, and the estimated battery internal resistance R.sub.n. The SOC.sub.nis estimated by integrating the estimated charging/discharging current I.sub.n. If the number m of estimations of the SOC.sub.nis m=10, the number m of estimations is changed to 0. The charging/discharging current i.sub.nis measured by a current sensor. The battery internal resistance R.sub.nis calculated from the battery voltage V.sub.nand the charging/discharging current i.sub.n. The battery temperature T.sub.nis also measured, and the T-R correlation map is corrected