To correctly determine abnormality of a set battery by simple calculation regardless of the pattern of changing and discharging or temperature variation.

A difference ΔV between the maximum value and the minimum value in the voltages of respective unit cells constituting a set battery is determined (S120, S130), and a plurality of the differences are coupled with a current I flowing through the set battery and memorized as coupled values (S140). When the relationship between the difference ΔV and the current I is considered as a linear relationship, since its gradient ΔR is equivalent to the difference of internal resistance, the gradient ΔR is determined by a least squares technique (S170), and the abnormality of the set battery is determined by the magnitude of the gradient ΔR (S200). The variation of an electromotive voltage due to the pattern of charging and discharging and the variation of the electromotive voltage due to temperature variation are taken into account because they are canceled when the difference ΔV of the voltage is obtained, so that the abnormality of the set battery can correctly by determined.