Error in an electromotive force Ve of a secondary battery calculated on the basis of a polarized voltage Vp is reduced. A polarized voltage computing section acquires multiple data pairs of a current I flowing to the secondary battery and a terminal voltage V of the secondary battery with respect to the current I, calculates an integrated capacity Q by integrating the acquired current I over a predetermined period, computes the polarized voltage Vp of the secondary battery on the basis of the integrated capacity Q, a no-load voltage computing section computes a no-load voltage V0 on the basis of multiple data pairs, a subtracter computes the electromotive force Ve of the secondary battery by subtracting the polarized voltage Vp from the no-load voltage V0, an electromotive force correcting section performs correction with respect to a presently computed electromotive force Ve so that a change amount between a previously computed electromotive force Veb and the presently computed electromotive force Ve does not exceed a predetermined limiting value Vt, and an SOC estimating section estimates the state of charge of the secondary battery on the basis of the post-correction electromotive force Ve'.