A film forming apparatus includes a mounting table on which a substrate is mounted; a ceiling plate facing the mounting table, the ceiling plate defining a processing space between the ceiling plate and mounting table; and a gas supply mechanism configured to supply a source gas to the processing space horizontally with respect to the substrate. A facing surface of the ceiling plate or a facing surface of the mounting table is inclined such that a gap between the facing surfaces of the mounting table and the ceiling plate becomes wider at a downstream side than at an upstream side in a flow direction of the source gas.