In a steam-water separator (45), a swirl vane (55) is fixed inside the riser (52) having a vertical part (53) and a curved part (54), an annular downcomer space (58) is formed by providing a downcomer barrel (56) outside the vertical part (53) of the riser (52), a deck plate (60) is arranged above the riser (52) and the downcomer barrel (56) with a predetermined space therefrom, an orifice (61) and vents (62) are formed, and a plurality of horizontal slits (64) are formed as a liquid film flow discharging member on the outer side of the curving direction of the curved part (54), at the location between the curved part (54) of the riser (52) and the swirl vane (55). As a result, the thickness of a liquid film formed inside a steam-water riser pipe is made even, and an overflow of a liquid film flow is prevented, thereby enhancing steam-water separating efficiency.