Provided is a method of manufacturing a semiconductor device. The method includes: (a) forming an oxide film having a predetermined thickness on a substrate by alternately repeating: (a-1) forming a layer containing a predetermined element on the substrate by supplying a source gas containing the predetermined element into a process vessel accommodating the substrate and exhausting the source gas from the process vessel; and (a-2) changing the layer containing the predetermined element into an oxide layer by supplying an oxygen-containing gas and an hydrogen-containing gas into the process vessel, wherein inside of the process vessel is under a heated atmosphere having a pressure lower than an atmospheric pressure; and exhausting the oxygen-containing gas and the hydrogen-containing gas from the process vessel; and (b) modifying the oxide film formed on the substrate by supplying the oxygen-containing gas and the hydrogen-containing gas into the process vessel, wherein the inside of the process vessel is under the heated atmosphere having the pressure lower than the atmospheric pressure, and exhausting the oxygen-containing gas and the hydrogen-containing gas from the process vessel.