A concentration distribution in an active material of a battery unit is calculated by using a diffusion equation, and a first polarization elimination time taken for the concentration distribution in the active material to fall within an allowable range is calculated assuming that charge and discharge of the battery unit is not performed. A concentration distribution in an electrolyte of the battery unit is calculated by using a diffusion equation, and a second polarization elimination time taken for the concentration distribution in the electrolyte to fall within an allowable range is calculated assuming that the charge and the discharge of the battery unit is not performed. It is determined that polarization of the battery unit is eliminated when a time for which the charge and the discharge of the battery unit is not performed is longer than the longer one of the first polarization elimination time and the second polarization elimination time.