Disclosed is an aqueous solution of a chromium salt, in which the oxalic acid content is 8% by weight or less relative to chromium. In the aqueous solution of the chromium salt, the total organic carbon content is 4% by weight or less relative to chromium. The chromium salt is preferably a chromium chloride, a chromium phosphate, or a chromium nitrate. The chromium chloride preferably contains a basic chromium chloride represented by the composition formula Cr(OH).sub.xCl.sub.y (wherein 0