NOVELTY - The apparatus has substrates (20) which are housed in processing chamber (30). The substrates are provided with lamination film. A reaction tube (100) is provided to constitute processing chamber. A gas supply tube (300) is configured to introduce elemental selenium-containing gas or elemental sulfur-containing gas to processing chamber. A heating section (200) is provided to surround reaction tube. A coating film with void rate of 5-15% and comprising mixture of chromium oxide and silica is formed on surface exposed to the gas.    USE - Substrate processing apparatus for manufacturing selenide-based copper-indium-selenium (CIS) solar battery.    ADVANTAGE - The coating film is provided with specific void rate, and comprised of mixture of chromium oxide and silica formed on surface exposed to the gas, and number of glass substrates can be housed in reaction tube, such that thin, uniform and compact structural design of the processing apparatus can be achieved, and manufacturing cost of CIS solar battery can be reduced effectively. The molding of reaction tube can be facilitated, even if size of reaction tube is increased, and the reaction tube can be handled easily compared to quartz reaction tube compared with the quartz reaction tube, so that working and handling efficiency of the apparatus can be improved efficiently.    DETAILED DESCRIPTION - The apparatus has substrates which are housed in processing chamber. The substrates are provided with lamination film composed of copper-indium, copper-gallium and copper-indium-gallium. A reaction tube is provided to constitute processing chamber. A gas supply tube is configured to introduce elemental selenium-containing gas or elemental sulfur-containing gas to processing chamber. An exhaust tube (310) is configured to exhaust atmosphere in processing chamber. A heating section is provided to surround reaction tube. A coating film containing chromium oxide having composition of formula: CrxOy, where x, y are 1 or more and silica, is formed on the surface exposed to elemental selenium-containing gas or elemental sulfur-containing gas, out of the surface of the reaction tube on the processing chamber side. An INDEPENDENT CLAIM is included for method for forming coating film on surface of reaction tube.    DESCRIPTION OF DRAWING(S) - The drawing shows a side cross sectional view of the processing furnace.    Substrate (20)    Processing chamber (30)    Reaction tube (100)    Heating section (200)    Gas supply tube (300)    Exhaust tube (310)