Abstract: A substrate processing apparatus includes a reaction container for processing a substrate therein. The reaction container is provided with two open ends. The reaction container includes a central cylindrical body and two flanges respectively provided at the both open ends. The central cylindrical body includes at its both ends thin portion which are thinner than the central portion of the central cylindrical body, the flanges are respectively provided with flange connecting portions which are thinner than the central portion of the central cylindrical body, and the thin portions of the central cylindrical body and the flange connecting portions are welded together.