A film forming apparatus includes: first and second source gas nozzles installed so as to extend in an arrangement direction of the substrates, each of the source gas nozzles including a plurality of gas ejection holes formed to eject the source gas toward central regions of the substrates at height positions corresponding to gaps between the substrates; a reaction gas supply unit configured to supply the reaction gas into the reaction vessel; first and second source gas supply lines respectively connected to the first and second source gas nozzles; first and second tanks respectively installed on the first and source gas supply lines, and configured to accumulate the source gas in a pressurized state; valves respectively installed at upstream and downstream sides of the first tank and at upstream and downstream sides of the second tank; and an exhaust port configured to evacuate the interior of the reaction vessel.