Provided is a substrate processing apparatus. The substrate processing apparatus includes a reaction tube; a heating device configured to heat the reaction tube; and a manifold installed outward as compared with the heating device and made of a non-metallic material. A first thickness of the manifold defined in a direction perpendicular to a center axis of the reaction tube is greater than a second thickness of the manifold defined at a position adjacent to the reaction tube in a direction parallel to the center axis of the reaction tube. The manifold includes a protrusion part of which at least a portion protrudes inward more than an inner wall of the reaction tube, and a gas supply unit disposed at at least the protrusion part for supplying gas to an inside of the reaction tube.