Provided is a technique capable of suppressing the occurrence of by-products by suppressing adhesion of the by-products. A substrate processing apparatus includes: a reaction tube where a substrate is processed; a furnace opening unit disposed at a lower end of the reaction tube and having an upper surface and an inner circumferential surface, the furnace opening unit including: a concave portion disposed on the upper surface; and a convex portion having at least one notch connecting the concave portion to the inner circumferential surface; a cover covering at least the inner circumferential surface with a predetermined gap therebetween; and a gas supply unit configured to supply a gas to the concave portion.