A method includes forming a film of a compound on an inner wall of a gas supply line by polymerization of a first compound and a second compound by controlling a temperature of the gas supply line to a first temperature at which the first compound and the second compound are polymerized in a state where a first gas containing the first compound and a second gas containing the second compound are supplied to the gas supply line, and removing the film by controlling the temperature of the gas supply line to a second temperature at which the film is depolymerized after predetermined processing is performed on a target object in a processing chamber by a processing gas supplied into the processing chamber through the gas supply line having the film. The first compound is isocyanate. The second compound is amine or a compound having a hydroxyl group.