A pressure vessel for a turbine is disclosed. A cylindrical shape of the pressure vessel is divided into two parts in a radial direction of the pressure vessel and the pressure vessel is connected in the cylindrical shape via flanges protruding outward in the radial direction of the pressure vessel at both divided ends of the pressure vessel. Increased thickness portions are defined in a portion excluding the divided ends and circular arc center portions farthest from the divided ends. The portion is between the divided ends and the circular arc center portions and the increased thickness portions increase radial thickness of the portion.