PURPOSE: To enable uniform treatment with decreased fluctuations in carburizing depth and nitriding depth by constituting plural pieces of discharge ports provided to a furnace shell in such a manner that said ports can be discretely opened and closed by stop valves.

CONSTITUTION: The stop valves 13aW13d of an electromagnetic operation type are connected respectively to the discharge ports 12aW12d provided to the shell 1 of, for example, an ion carburizing furnace 22 and the discharge sides of the valves 13 are connected to a vacuum pump 10. An object 5 to be treated is first imposed on a hearth and a furnace cap is closed, then the inside of the furnace is evacuated to a prescribed vacuum degree by the pump 10. The object 5 is thereafter heated to a prescribed temp. by a heater 7 and gaseous Ar and H2 are supplied from a manihold 8 into the furnace. The surface of the object 5 is cleaned by the glow discharge between an anode 2 and the object 5 on a cathode 3. Gaseous CH is then supplied similarly into the furnace to effect the ion carburizing treatment. The valves 13aW13d are opened sequentially by each piece for the prescribed time (e.g.: 2 minutes) to switch the flow of the treating gas successively in 4 ways during this treatment.