A semiconductor device (having an interlayer insulating film) which is sufficiently low in the dielectric constant and high in the mechanical strength is provided.A manufacturing method of a semiconductor device includes: a step of forming a dielectric thin film in which a plurality of pores are arranged around a skeleton mainly made of a Si--O bond, on a surface of a semiconductor substrate on which a desired element region is formed; a step of applying patterning on a surface of the dielectric thin film through a mask; and a step of bringing a gas containing at least one kind of tetramethylcyclotetrasiloxane (TMCTS), hexamethyldisilazane (HMDS) and trimethylchlorosilane (TMCS) molecules into contact with the patterned surface of the dielectric thin film.