A film forming apparatus comprises a processing chamber for holding therein a to-be-processed substrate, a first gas supplying means for supplying into the processing chamber a first vapor source including a metal alkoxide having a tertiary butoxyl group as a ligand, and a second gas supplying means for supplying into the processing chamber a second vapor source including a silicon alkoxide source, wherein the first gas supplying means and the second gas supplying means are connected to a pre-reaction means for causing pre-reactions of the first vapor source and the second vapor source, and the film forming apparatus is configured to supply the first vapor source and the second vapor source after the pre-reactions into the processing chamber.