Abstract: There is provided a technique capable of uniformly supplying a gas with respect to a surface of a substrate when the substrate is processed. According to one aspect thereof, there is provided a substrate processing apparatus including: a reaction tube; and a gas supply nozzle for supplying a gas to a substrate supported by a substrate support in the reaction tube along a direction parallel to a substrate surface. The gas supply nozzle is provided with a gas ejection port including an edge vicinity portion and a central portion defined along the direction parallel to the surface of the substrate. An opening dimension of the edge vicinity portion of the gas ejection port measured along a direction orthogonal to the direction parallel to the surface of the substrate is greater than an opening dimension of the central portion of the gas ejection port measured along the same direction.