A steam generator comprising: a barrel section having a sealed hollow shape; a tube bundle outer casing being disposed with a predetermined gap provided between the tube bundle outer casing and an inner wall of the barrel section; a high temperature-side heat transfer tube bundle and a low temperature-side heat transfer tube bundle including a plurality of heat transfer tubes disposed in the tube bundle outer casing and having an inverse U-shape; a tube plate being secured to the barrel section on a side near a water chamber (13, 14) so as to secure ends of the plurality of heat transfer tubes; a plurality of tube support plates (6A, 6B, 6C, 6D, 6E, 6F, 6G) for supporting the heat transfer tubes; a steam-water separator being disposed in an upper portion of the barrel section so as to separate steam and a recirculating secondary coolant; a water supply channel for supplying a secondary coolant from the outside of the steam generator to the tube bundle outer casing with the secondary coolant being isolated from the recirculating secondary coolant; a recirculating secondary coolant injection section for injecting the recirculating secondary coolant toward the high temperature-side heat transfer tube bundle; and a supplied secondary coolant injection section for injecting the secondary coolant supplied from the water supply channel toward the low temperature-side heat transfer tube bundle, the recirculating secondary coolant injection section and the supplied secondary coolant injection section being disposed in the tube bundle outer casing on a side near the tube plate, characterized in that an opening area of the recirculating secondary coolant injection section is different from an opening area of the supplied secondary coolant injection section, so that, of the secondary coolant and the recirculating secondary coolant, the coolant having a higher relative flow rate than the other flows at a lower velocity. a barrel section having a sealed hollow shape; a tube bundle outer casing being disposed with a predetermined gap provided between the tube bundle outer casing and an inner wall of the barrel section; a high temperature-side heat transfer tube bundle and a low temperature-side heat transfer tube bundle including a plurality of heat transfer tubes disposed in the tube bundle outer casing and having an inverse U-shape; a tube plate being secured to the barrel section on a side near a water chamber (13, 14) so as to secure ends of the plurality of heat transfer tubes; a plurality of tube support plates (6A, 6B, 6C, 6D, 6E, 6F, 6G) for supporting the heat transfer tubes; a steam-water separator being disposed in an upper portion of the barrel section so as to separate steam and a recirculating secondary coolant; a water supply channel for supplying a secondary coolant from the outside of the steam generator to the tube bundle outer casing with the secondary coolant being isolated from the recirculating secondary coolant; a recirculating secondary coolant injection section for injecting the recirculating secondary coolant toward the high temperature-side heat transfer tube bundle; and a supplied secondary coolant injection section for injecting the secondary coolant supplied from the water supply channel toward the low temperature-side heat transfer tube bundle, the recirculating secondary coolant injection section and the supplied secondary coolant injection section being disposed in the tube bundle outer casing on a side near the tube plate, characterized in that