After forming a resist film by coating a semiconductor substrate with a resist, pattern exposure is conducted by irradiating the resist film with ArF excimer laser with a mask used. A silylation agent of 4-dimethylsiloxy-3-penten-2-one is supplied onto the surface of the resist film having been subjected to the pattern exposure, thereby forming a silylated layer in an unexposed portion of the resist film. The resist film is etched by using the silylated layer as a mask, so as to remove an exposed portion of the resist film. Thus, a resist pattern can be formed out of the resist film.