The invention provides a method for subjecting laminated thin films disposed below a photoresist mask pattern to plasma processing, wherein the roughness on the side walls of the formed pattern is reduced, and the LER and LWR are reduced. When etching a material to be processed to form a gate electrode including thin films such as a gate insulating film 205, a conducting layer 204, a mask layer 203 and an antireflection film 202 laminated on a semiconductor substrate 206 and a photoresist mask pattern 201 disposed on the antireflection film, prior to etching the mask pattern 201, plasma is generated from nitrogen gas or a mixed gas including nitrogen gas and deposition gas to subject the mask pattern 201 to a plasma curing process so as to reduce the roughness on the surface and side walls of the mask pattern 201, and then the laminated thin films 202, 203 and 204 disposed below the mask pattern 201 are subjected to a plasma etching process.