A film forming apparatus for performing a film forming process by sequentially supplying a plurality of reactant gases to a substrate and supplying a replacement gas includes a mounting table configured to mount thereon a substrate, and a shower head having a flat surface facing the mounting table and a plurality of gas supply opening. An annular protrusion is provided at the shower head to form a gap between the annular protrusion and a top surface of the mounting table. A plurality of gas supply units is provided at a ceiling portion at an upper side of the shower head. Each gas supply unit has gas discharge openings formed along a circumferential direction. The diffusion space is disposed such that an outer periphery of the diffusion space is located at an inner side of an outer periphery of the substrate mounted on the mounting table in a plan view.