PURPOSE: To obtain a steam generator for a pressurized water reactor having high intactness and a long life in which the centrifugal force of high speed water flow at the turning point of flow direction of water in a primary cooling system is lowered, vibration due to turbulent flow is reduced.

CONSTITUTION: At the bottom of the case 1 in a steam generator, a primary coolant water inlet/outlet consisting of a primary coolant water inlet 2a where the primary coolant heated in the reactor flows in and a primary coolant water outlet 2b where the primary coolant after transferring heat to the secondary coolant water is returned to the reactor, is provided. In the case 1, a plurality of thin tubes 3 which consist of going thin tubes 3a and returning thin tubes 3b and connect the primary coolant inlet 2a and the primary coolant outlet 2b. In the flow direction changing part between the going thin tubes 3a and the returning thin tubes 3b, a reservoir 7 with larger cross sectional area than that of the thin tubes 3 is provided. In the case 1, a secondary coolant inlet 4 to supply secondary coolant water to around the thin tubes 3 and a steam outlet 5 to exhaust steam generated by heating of the secondary coolant water are provided.