In a semiconductor device fabricating process, a chemical amplification resist layer is formed on an insulating film formed on a semiconductor substrate, and the chemical amplification resist layer is patterned to form an opening. The insulating film formed on the semiconductor substrate is wet-etched using the patterned chemical amplification resist layer as a mask. Before the wet-etching is carried out, a surface treatment is conducted for the patterned chemical amplification resist layer to form an insoluble layer at a surface of the patterned chemical amplification resist layer, thereby to elevate a wet-etching-resistance of the patterned chemical amplification resist layer. Thus, deformation of a resist pattern formed of the patterned chemical amplification resist layer is prevented in the wet etching process, so that an opening pattern of a desired shape is formed in the insulating film.