If a particular selection condition is satisfied, a non-load voltage calculation unit (105) calculates a no-load voltage Vsep as a voltage piece when the current in approximate straight line obtained by statistical processing using the method of least squares for a plurality of sets of data containing current data I(n) and voltage data V(n) is zero. If a particular current condition or voltage condition is continuously satisfied for a predetermined time, an open-circuit voltage calculation unit (106) calculates a secondary cell terminal voltage as an open-circuit voltage Voc and a voltage-at-zero-current storing unit (108) stores voltage-at-zero-current Vzo calculated by a voltage-at-zero-current calculation unit (107). By using a predetermined voltage change amount adjustment constant .DELTA.Vbc/adjustment coefficient Kb, an electromotive force change constant Keq, and a polarization voltage generation constant Kpol, an estimated charge/discharge electric amount calculation unit (118A) calculates an estimated charge/discharge electricity amount .DELTA.Qe as a function of the change amount .DELTA.Vzo of a voltage-at-zero-current Vzo. It is possible to estimate charge/discharge electricity amount without being affected by the current measurement error.