The present invention provides an apparatus that can estimate the charge/discharge electricity amount without being affected by current measurement error. If specific selection conditions are met, a no-load voltage calculation part (105) takes a plurality of pairs of data consisting of current data I(n) and voltage data V(n) corresponding to the current data and calculates a no-load voltage V.sub.sep as the voltage intercept at a current of zero in a straight-line approximation obtained by statistical processing such as regression analysis using a least squares method with respect to the plurality of pairs of data. In addition, if specific current conditions continue to be met for a certain amount of time, an open circuit voltage calculation part (106) calculates the terminal voltage of the secondary battery as the open circuit voltage V.sub.oc. An estimated charge/discharge electricity amount calculation part (114) uses a preset change-in-voltage adjustment constant .DELTA.V.sub.bc/adjustment coefficient K.sub.b, change-in-electromotive-force constant K.sub.eq and polarization voltage generation constant K.sub.pol to calculate the estimated charge/discharge electricity amount .DELTA.Q.sub.e as a function of the change .DELTA.V.sub.b in the no-load voltage or the open circuit voltage over a predetermined period of time.