Provided are a semiconductor device manufacturing method and a substrate processing apparatus. The method comprise: a first process of forming a film containing a predetermined element on a substrate by supplying a source gas containing the predetermined element to a substrate processing chamber in which the substrate is accommodated; a second process of removing the source gas remaining in the substrate processing chamber by supplying an inert gas to the substrate processing chamber; a third process of modifying the predetermined element-containing film formed in the first process by supplying a modification gas that reacts with the predetermined element to the substrate processing chamber; a fourth process of removing the modification gas remaining in the substrate processing chamber by supplying an inert gas to the substrate processing chamber; and a filling process of filling an inert gas in a gas tank connected to the substrate processing chamber.