Abstract: A lead acid battery is described that makes it possible to suppress an increase in internal resistance and to accurately determine the state of charge or the state of degradation by a method of measuring the internal resistance. The lead acid battery includes an electrode plate group in which a plurality of positive electrode plates having a positive active material containing lead dioxide and a plurality of negative electrode plates having a negative active material containing metallic lead are alternately stacked with separators interposed therebetween. The electrode plate group is immersed in an electrolyte. The flatness of the positive electrode plates after chemical conversion is equal to or less than 4.