The present invention relates to a cleaning agent for a semiconductor substrate having a copper wiring film or a copper alloy wiring film, and a cobalt-containing film to be used in a post-process of a chemical mechanical polishing process, comprising (A) an organic acid represented by general formula described in the present specification, (B) amines selected from the group consisting of (B-1) diamines, (B-2) amidines, (B-3) azoles, and (B-4) pyrazines or pyrimidines, represented by general formulae described in the present specification, (C) a hydroxylamine derivative, and (D) an oxygen scavenger represented by general formula described in the present specification, and being an aqueous solution having a pH of 10 or higher; and a processing method for the surface of a semiconductor substrate, having a copper wiring film or a copper alloy wiring film, and a cobalt-containing film, which comprises using the cleaning agent.