Disclosed is a producing method of a semiconductor device, including: loading a substrate into a reaction tube; oxidizing the substrate under an atmospheric pressure by supplying a plurality of kinds of gases which react with each other and an inert gas into the reaction tube; and unloading, from the reaction tube, the substrate after the oxidizing, wherein in the oxidizing, a flow rate of the inert gas is varied in accordance with a variation of the atmospheric pressure to keep constant a partial pressure of an oxidizing gas or partial pressures of oxidizing gases in the reaction tube, and the flow rate of the inert gas is calculated based on a pre-calculated flow rate of a gas or pre-calculated flow rates of gases produced by reaction of the plurality of gases and a pre-calculated flow rate of a gas which is not consumed by the reaction and which remains or pre-calculated flow rates of gases which are not consumed by the reaction and which remain.