A cleaning agent for use in the manufacture of a semiconductor device comprising an aqueous solution containing a quarternary ammonium salt and a fluoro compound, or an aqueous solution containing a quarternary ammonium salt and a fluoro compound, as well as an organic solvent selected from the group consisting of amides, lactones, nitriles, alcohols and esters. In the semiconductor device manufacturing process, after forming a mask with a photoresist, a wiring structure is formed by dry etching of a conductive layer, wherein a protecting deposition film has been formed on side walls of the conductive layer. Use of the cleaning agent enables the protecting deposition film to be removed in a highly reliable manner with the surface of the conductive layer being decontaminated and cleaned such that no corrosion of the conductive layer occurs.