An amorphous silicon film formation method includes transferring a base in a process chamber, heating the base in the process chamber, setting a process pressure inside the process chamber, forming a seed layer on a surface of the base by flowing aminosilane-based gas in the process chamber under a process condition in which the aminosilane-based gas is not thermally decomposed and adsorbing the aminosilane-based gas onto the surface of the base, the process condition having a first temperature, and forming an amorphous silicon film on the seed layer by heating the base at a second temperature higher than the first temperature, flowing silane-based gas containing no amino group in the process chamber, and thermally decomposing the silane-based gas containing no amino group, wherein the forming of the seed layer and the forming of the amorphous silicon film are successively performed in the process chamber.