A microcomputer in a device body operates so as to store every update of a maximum discharge current value (IMAX (A)) of a secondary battery (the whole plurality of cells), and operates so as to indicate the ratio value of the remaining capacity defined in a remaining capacity definition table with respect to a discharge voltage value (V (V)) of the secondary battery while reducing the ratio value in accordance with the maximum discharge current value (IMAX (A)) when obtaining the discharge voltage value (V (V)) from a protection circuit.