At a low temperature of 500° C. to 700° C., the concentration of atomic oxygen is controlled in a wafer stacked direction, and the thickness distribution of oxide films is kept uniform in the wafer stacked direction. A semiconductor device manufacturing method includes a process of oxidizing substrates by supplying oxygen-containing gas and hydrogen-containing gas through a mixing part from an end side of a substrate arrangement region where the substrates are arranged inside the process chamber so that the gases flow toward the other end side of the substrate arrangement region, and supplying hydrogen-containing gas from mid-flow locations corresponding to the substrate arrangement region. The oxygen-containing gas and the hydrogen-containing gas reacts with each other in the mixing part to produce an oxidation species containing atomic oxygen, and the oxidation species has a maximum concentration at an ejection hole through which the oxidation species is ejected from the mixing part into the process chamber.