A flattening method, by utilizing the advantages of the CARE method and making up for the disadvantages, can perform removal processing of a surface of a workpiece at a sufficient processing rate and can provide a processed surface having enhanced flatness without leaving damage in the processed surface. A flattening method comprises at least two surface removal steps and at least two cleaning steps, the final surface removal step being a catalyst-referred etching step comprising immersing a workpiece in a processing solution containing at least one of hydrohalic acid, hydrogen peroxide water and ozone water, and bringing a surface of a catalyst platen into contact with or close proximity to a surface to be processed of the workpiece to process the surface, said catalyst platen having in a surface a catalyst selected from the group consisting of platinum, gold, a ceramic solid catalyst, a transition metal, glass, and an acidic or basic solid catalyst.