NOVELTY - The substrate processing apparatus has reaction tube that constitutes processing chamber. A gas supply tube introduces elemental selenium-containing gas or elemental sulfur-containing gas into processing chamber. An exhaust tube is configured to exhaust atmosphere in the processing chamber. A heating section surrounds the reaction tube. The base (101) of reaction tube is made of metal material e.g. stainless. A porous coating film (102) is composed of ceramics or carbon and formed on surface exposed to elemental selenium- or sulfur-containing gas in processing chamber side of reaction tube.    USE - Substrate processing apparatus for forming light absorbing layer of selenide-based CIS solar battery.    ADVANTAGE - Reduces manufacturing cost of the CIS-based solar battery since number of glass substrate that can be housed in reaction tube is increased and number of glass substrates that can be processed at once is increased by using metal material such as stainless steel for the base of reaction tube and arranging the cassettes holding the glass substrates side by side in the direction parallel to the surface of glass substrates. The generation of crack on coating film is suppressed to minimum even if heat treatment is repeatedly performed since coating film is formed by porous film. The reaction tube is easy to be handled compared with quartz reaction tube and size of reaction tube is increased by using metal material such as stainless steel as base of reaction tube. The processing is performed using highly corrosive selenium source by forming the coating film with high selenization resistance on the base of reaction tube, thus reduces manufacturing cost of the CIS-based solar battery. Suppresses peel-off of the coating film caused by deviation of the coefficient of linear expansion between base and coating film since coating film is formed in porous shape. The cycle of maintenance is made large since deviation of the coefficient of linear expansion between base and coating film is set to preferably 20 percent or less, and more preferably 5 percent or less.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a method for manufacturing a CIS-based solar battery; and    (2) a method for manufacturing substrates.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the coating film.    Base (101)    Coating film (102)