An object of the present invention is to provide a film formation technique having high productivity by realizing a foundation layer having excellent crystallinity with a small film thickness of about 2 μm. An embodiment of the present invention relates to a film forming method which includes the step of forming a buffer layer by sputtering on a sapphire substrate held by a substrate holder. The buffer layer includes an epitaxial film having a wurtzite structure prepared by adding at least one substance selected from the group consisting of C, Si, Ge, Mg, Zn, Mn, and Cr to AlxGa1−xN (where 0≤x≤1).