A surface-treated steel sheet for battery cases is provided which comprises a nickel-cobalt alloy layer formed at the outermost surface of a plane to be an inner surface of a battery case. When X-ray diffraction measurement using CuK± as a radiation source is performed for the nickel-cobalt alloy layer, an intensity ratio I A /I B =0.01 to 0.9. The intensity ratio I A /I B is a ratio of an intensity I A of a peak present at a diffraction angle 2¸ within a range of 41° or more and less than 43° to an intensity I B of a peak present at a diffraction angle 2¸ within a range of 43° or more and 45° or less.
