The invention relates to an estimated charging amount calculator of a rechargeable battery. A charge/discharge current Iij where an absolute value of a difference of an estimated value (estimated cell voltage Vije(n)) of a terminal voltage of a battery cell and a detected value (cell voltage Vij(n)) that becomes equal to or less than a prescribed value DeltaVth is searched by Newton's method. The charge/discharge current Iij searched in this way is equalized in all the battery cells. A charging rate SOCij(n) of the battery cell is calculated by an integration calculation of a calculated average value Ia (n).