The present disclosure provides a technique including a method of manufacturing a semiconductor device, which is capable of improving the characteristics of a film formed on a substrate. The method of manufacturing a semiconductor device may include: (a) forming a first film containing a predetermined element, oxygen, carbon and nitrogen on a substrate; and (b) forming a second film thinner than the first film on a top surface of the first film, the second film having an oxygen concentration lower than an oxygen concentration of the first film or having oxygen and carbon concentrations lower than oxygen and carbon concentrations of the first film.