A method for determining deterioration of accumulator battery hooked up with loads in a system based on results of measuring internal resistances of an accumulator battery, the method comprising the steps of; predetermining as a specified temperature a temperature at which the deterioration of the accumulator battery is determined; calculating in advance temperature correction coefficients of the internal resistances from changes of the internal resistances depending on temperatures; predetermining resistance-voltage conversion factors to convert between the internal resistances at the specified temperature and terminal discharge voltages of the accumulator battery which are obtained at the specified temperature under a condition of flowing predetermined discharge currents from the accumulator battery; measuring the internal resistances of the accumulator battery and temperature of the accumulator battery at an internal resistance measurement; converting the measured internal resistance values into the internal resistance values at the specified temperature with use of the temperature correction coefficients of the internal resistances; converting the internal resistance values at the specified temperature into the terminal discharge voltage values of the accumulator battery at the specified temperature with use of the resistance-voltage conversion factors; and determining whether the accumulator battery is deteriorated or not by means of comparison of the terminal discharge voltage values of the accumulator battery at the specified temperature and a predetermined threshold value as a deterioration judgment standard.