A steam generator is provided in which the total heat transfer efficiency can be improved by suppressing a reduction in the amount of heat exchange at an inlet side of heat transfer tubes while maintaining the effect of increasing the amount of heat exchange at an outlet side thereof. A steam generator (1) includes a heat transfer tube group (15) formed of a plurality of heat transfer tubes (13) secured to a tube plate (11); an annular channel (23) that is formed so as to cover the circumference of the heat transfer tube group (15) and that has an opening (25) for communicating with the heat transfer tube group (15) at a lower portion thereof; a water supply box (29) that is disposed at an upper portion of the annular channel (23) and that supplies water to a cold-side portion of the heat transfer tubes (13); and a steam/water separator (31) that is disposed above the heat transfer tube group (15) and that separates water heated while passing from the annular passage (23) along the circumference of the heat transfer tubes (13) into steam and hot water, in which a porous plate (35) for removing air bubbles is provided at an upper portion of the annular channel (23) in the hot-side portion of the heat transfer tubes (13).
