Provided is a method of manufacturing a semiconductor device. The method includes (a) loading a substrate into a processing chamber; (b) starting a supply of a first processing gas into the processing chamber; (c) starting a supply of a second processing gas into the processing chamber during the supply of the first processing gas; (d) stopping the supply of the second processing gas during the supply of the first processing gas; (e) stopping the supply of the first processing gas after performing the step (d); (f) removing the first processing gas and the second processing gas remaining after performing the step (e) from the processing chamber; and (g) unloading the substrate from the processing chamber.