To prevent the occurrence of thermal damage through promotion of mixture of feed water and circulation hot water when heat transfer performance of a tubular heat-exchange type vertical steam generator is improved.

A vertical steam generator comprises a drum 31 to integrally form a water chamber at a lower part; a number of inverted U-shaped heat- transfer pipes to form a heat-transfer pipe group arranged in the drum 31; a wrapper structure 43 arranged in the drum 31 in a state to surround the heat-transfer pipe group and form a downcomer 49 in coordination with the drum 31; a feed water nozzle 45 positioned facing the lower part of the wrapper structure 43 and constituted integrally with the drum 31; a steam separator arranged at the upper part of the wrapper structure 43; a mixing plate 61 positioned below the feed water nozzle 45 and arranged in the downcomer 49; and a porous plate 63 positioned between a tube plate 33 partitioning the interior of the drum 31 from a water chamber and the mixing plate 61 and arranged in the downcomer 49.