According to the present invention, flatness of a thin film formed on a substrate is improved without generating particles and lowering productivity. A method of manufacturing a semiconductor device includes a first thin film layer forming step A and a second thin film layer forming step B. In the first thin film layer forming step A, on the way of heating and raising the temperature of the substrate up to a film-forming temperature, a film-forming source supply in which an organic source gas is made adhere onto the substrate in yet unreacted state is performed (202), and thereafter, a RPO process (Remote Plasma Oxidation) in which an oxygen radical is supplied onto the substrate to form a first thin film layer is performed (203). In this first thin film layer forming step A, it is preferable to repeat the film-forming source supply onto the substrate and the RPO process more than once. In the second thin film layer forming step B, after the source gas is supplied onto the substrate by a thermal CVD method to perform a film-forming process (205) after raising the temperature of the substrate to the film-forming temperature, the RPO process is performed to form a second thin film layer on the first thin film layer with a predetermined film thickness (206).