The present invention provides a separator for a fuel cell comprising a resin and a conductive material as constituting components, and sulfonic acid groups imparted to at least one portion at the surface of gas channels by a treatment using a sulfuric acid-containing gas, wherein the resin and the sulfonic acid groups, which are on the surface of the gas channels, are bonded, and a ratio of sulfur atoms in the sulfonic acid groups at the surface of the gas channels as determined by energy-dispersive X-ray spectroscopy is in a range from 0.1 to 4.0 at %, and a method for producing the separator. The separator for a fuel cell of the present invention is excellent in the wettability to water, since sulfonic acid groups are imparted to the resin at the surface of gas channels in the sulfuric acid-containing gas.