Abstract: In a method for processing a substrate, a plurality of substrates maintained in a boat are loaded into a cylindrical inner tube disposed in a cylindrical outer tube. A processing gas is supplied into a process room, and thereafter the substrates are batch-processed with the processing gas evacuated through an exhaust path formed between the inner tube and the outer tube, wherein nitrogen gas is supplied to a surface region of the ceiling of the outer tube during a film forming process of the substrates, thereby the processing gas ascended through the process room is prevented from coming into contact with the ceiling of the outer tube by the nitrogen gas covering thereat. Accordingly, products and/or by-products of the film forming gas is prevented from being adhered thereto, thereby formation of contaminants due to the deterioration of the deposition of the products and the by-products thereof can be eliminated/reduced.