Vertical sectional shapes of portions in which condenser tubes of the upper tube bundle 51 and the lower tube bundle 52 are arranged, in vertical sections of the upper tube bundle 51 and the lower tube bundle 52, are formed to be approximately U-shapes, and a noncondensing air ejection duct 11 is provided to be positioned on a central joint portion of the U-shape of the upper tube portion 51 in an upstream side where circulating water flows first. At a portion in which the condenser tubes are not arranged between the upper and lower tube bundles, steam flow prevention plates 53 are provided to be positioned at both right and left sides of the noncondensing air ejection duct 11.