In a method of forming a silicon oxide film, a target substrate that has a silicon layer on a surface is loaded into a process area within a reaction container, while setting the process area to have a loading temperature of 400° C. or less. Then, the process area that accommodates the target substrate is heated, from the loading temperature to a process temperature of 650° C. or more. Water vapor is supplied into the reaction container during said heating the process area, while setting the water vapor to have a first concentration in an atmosphere of the process area, and setting the process area to have a first reduced pressure. After said heating the process area to the process temperature, an oxidation gas is supplied into the reaction container, thereby oxidizing the silicon layer to form a silicon oxide film.