A film forming apparatus sequentially supplies a raw material gas of a compound containing chlorine and an element other than the chlorine, and a first reaction to form a fil. The film forming apparatus includes a rotary table, a raw material gas ejection port configured to eject the raw material gas to a first region, a reaction gas supply part configured to supply, to a second region, a first reaction gas and a second reaction gas that reacts with chlorine to generate a third reaction product, in order to prevent a second reaction product from being generated due to a reaction of the chlorine remaining in the vacuum container with air when performing the opening-to-air. The film forming apparatus further includes an atmosphere separation part, a first exhaust port and a second exhaust port, and a controller.