A thin stainless steel sheet used for the separator of a solid polymer fuel cell, wherein: in a surface layer up to a depth of 0.2 t (t being the total thickness of the steel sheet) from at least one surface, the volume fraction of M2B-type boride metal deposits is at least 8%, and the volume-based particle diameter frequency distribution for the M2B-type boride metal deposits with a sphere equivalent diameter of 6.72 µm or less is at least 99.5%; and the apparent density of the steel sheet when the true density is 1 is at least 0.995. A solid polymer fuel cell using said thin stainless steel sheet as the separator has excellent battery performance without applying high cost surface treatments such as expensive metal plating because of the reduction in surface contact resistance.
