NOVELTY - The furnace (1) has a heating mechanism (30) e.g. carbon heater or halogen lamp heater, fixed and in contact with an outer wall (21) of an inner body (20). A controller (50) is provided outside the outer body for controlling supply amount of gas into a gas circulation chamber (204) and the supply amount another gas into a reaction chamber (205) through a gas supplying mechanism (40) so as to form first pressure in the gas circulation chamber and second pressure in the reaction chamber. The outer body is made of steel or stainless steel such as SUS304 and SUS316.    USE - Multi-stage heat treating furnace for a gas reaction for producing copper indium gallium diselenide solar cells. Uses included but are not limited to a nitrogen, argon, hydrogen, nitrogen, hydrogen selenide, and hydrogen sulfide    ADVANTAGE - The gas circulation chamber in the heat treating furnace simultaneously introduces cooling gaseous nitrogen into the reaction chamber inside the inner body and the gas circulation chamber arranged between the inner body and the outer body so as to prevent formation of temperature gradient in the wall of inner body, and effectively protect the wall of inner body from chapping or peeling. The furnace is designed such that a safety gate door is provided to effectively protect the operator from danger of pressure imbalance inside the heat treating furnace so as to improve operational safety. The furnace is designed such that the reaction rate and uniformity are improved and the waste of reaction gas is reduced. The furnace arrangement enables the effective control of the gas inflow to improve the safety and efficiency of the thin film formation. The furnace saves cost of facility and transportation processes. The furnace has better reliability.    DETAILED DESCRIPTION - The inner body is made of quartz or silicon dioxide.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a heat treating furnace.    Multi-stage heat treating furnace (1)    Inner body (20)    Outer wall (21)    Heating mechanism (30)    Gas supplying mechanism (40)    Controller (50)    Gas circulation chamber (204)    Reaction chamber (205)