A deposition method includes causing aminosilane gas to be adsorbed on a substrate in which a recessed portion is formed on a surface of the substrate; causing a first silicon oxide film to be stacked on the substrate by supplying oxidation gas to the substrate to oxidize the aminosilane gas adsorbed on the substrate; and performing a reforming process on the first silicon oxide film by activating, by plasma, a first mixed gas including helium and oxygen, and supplying the first mixed gas to the first silicon oxide film.