A stable and minute processing method of a thin film is provided. Further, a miniaturized semiconductor device is provided. A method for processing a thin film includes the following steps: forming a film to be processed over a formation surface; forming an organic coating film over the film to be processed; forming a resist film over the organic coating film; exposing the resist film to light or an electron beam; removing part of the resist film by development to expose part of the organic coating film; depositing an organic material layer on the top surface and a side surface of the resist film by plasma treatment; etching part of the organic coating film using the resist film and the organic material layer as masks to expose part of the film to be processed; and etching part of the film to be processed using the resist film and the organic material layer as masks.