To improve working efficiency by efficiently plastically deforming a workpiece of a nonferrous metal such as Al, Ti, Mg or an alloy of each of these metals.

First, an oxide film existing on the surface of a nonferrous metallic workpiece is removed (removing process S1). Then, this surface from which the oxide film is removed is covered with a coating film that functions as an oxidation preventing film (coating process S2). Thus, the nonferrous metallic workpiece without the oxide film and formed with the coating film is plastically deformed in the working process S3.