A current Ibe and a voltage Vbe before a large current discharge in a secondary battery are stored in a memory. Plural voltage-current pairs are detected and stored in the memory during the large current discharge. Next, a regression curve (or a regression line) is obtained based on the plural voltage-current pairs stored in the memory using a well-known method. A discharge current Ip which exceeds a current-sensor detectable range is input in the regression curve to obtain a voltage Vp. A maximum discharge coordinate is set using the discharge current Ip and the voltage Vp. An internal resistance Rin of the secondary battery is calculated based on an inclination of a line that connects the coordinates (Ibe, Vbe) and (Ip, Vp).