A method of forming a cap film comprises a first polishing step of performing a polishing operation at selectivity of R1 (=removal rate for the cap film/removal rate for the insulating film), and a second polishing step of performing a polishing operation at selectivity of R2 (=removal rate for the cap film/removal rate for the insulating film). Each of the polishing operations is performed by using a slurry having the condition of R1>R2. By performing the polishing operations at different selectivity, the cap film free from problems such as dishing of the cap film and the residual cap film on side walls of a recess is formed. Consequently, a semiconductor device having an excellent RC characteristic can be provided.