9 and current data on a discharge current detected by the current sensor 7 as the automobile travels at a step s203 the voltage data as well as the current data are collected to be averaged each time when they amount to 100 in number concurrently the potential travel distance calculating file 36 reads the current data from the memory 31 storing them in correspondence to a temporal axis at a step s205 the averaged voltage and current data are processed for the voltage current approximate line calculating file 23 to determine an approximate straight line y ax b by a method of least squares and for the estimation capacity voltage calculating file 25 to substitute the reference discharge current i0 in the approximate line thereby obtaining a corresponding estimation capacity voltage which is stored at a step s207 the current accumulation interrupt decision file 32 reads the estimation capacity voltage checking for a capacity recovery caused by a restoration or a discharge interruption to make a decision whether or not it is effected e g such that it should have been effected if a restorating charge voltage is higher than a previous estimation capacity voltage upon an affirmative decision for a capacity recovery by a restoration or a discharge interruption the program flow of the current accumulation interruption decision file 32 goes to a step s209 where it interrupts the current accumulation then the flow goes to a step s211 for checking.