An apparatus estimates charge/discharge electricity amount without being affected by current measurement error. Under specific selection conditions, a no-load voltage calculation part uses pairs of data consisting of current data I(n) and voltage data V(n) corresponding to the current data to calculate 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. If specific current conditions exist for a certain amount of time, an open circuit voltage calculation part calculates the terminal voltage of the secondary battery as the open circuit voltage V.sub.oc. An estimated charge/discharge electricity amount calculation part 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 or open circuit voltage over a predetermined period of time.