A technique capable of suppressing the generation of foreign matter in a process container involves a method of manufacturing a semiconductor device including: (a) supplying a source gas to a substrate in a process container; (b) supplying an inert gas to an inner wall of an opening of the process container at a first flow rate while performing (a); (c) supplying a reactive gas to the substrate; and (d) supplying the inert gas to the inner wall at a second flow rate lower than the first flow rate while performing (c).