PURPOSE: To make the thicknesses of films which are formed on all treated objects by the chemical reaction of reactive gases on the surfaces of the treated objects uniform and improve a film forming process and a production yield.

CONSTITUTION: A wafer 1 is held by a chuck plate 2 provided in a treatment chamber 3. A gas nozzle through which reactive gases are supplied into the treatment chamber 3 is connected to gas bombs 22 and 23 through pipes 20 and 21. The reactive gases are supplied into the treatment chamber 3 before the first wafer 1 is subjected to a thin film treatment by controlling valves 24 and 27 provided in the pipes 20 and 21 with a controller 28 so as to have the atmosphere in the treatment chamber 3 equivalent to the thin film treatment atmosphere.