A gas removal system that removes a halogen gas remaining inside a processing chamber after executing a specific type of processing inside the processing chamber maintained in an airtight state with plasma obtained through discharge dissociation of the halogen gas supplied from a gas supply device comprises a pressure control device that controls the pressure inside the processing chamber, an air supply device that supplies the atmospheric air into the processing chamber after the pressure inside the processing chamber is lowered by the pressure control device, a control device that controls the air supply device and an evacuation device that evacuates a gas produced through a reaction of the halogen gas and the atmospheric air having occurred inside the processing chamber.