A once-through heat exchanger comprising: a plurality of tubes ; a plurality of heads connected to the tubes and configured to accommodate heated steam; a manifold connected to the heads and configured to accommodate heated steam; and a first link pipe and a second link pipe configured to connect the heads and the manifold , characterized in that : the first link pipe or the second link pipe comprises a variable pipe whose inner diameter gradually changes; wherein the first link pipe comprises a first connection link part extending from an associated one of the heads toward the manifold and a first inclined link part protruding obliquely from the first connection link part and coupled to the manifold ; wherein the first inclined link part comprises a large diameter portion having a larger outer diameter than the first connection link part , a small diameter portion having the same inner diameter as the first connection link part , and the variable pipe connecting the large diameter portion and the small diameter portion; and wherein the large diameter portion is connected to the manifold , and the inner diameter of the variable pipe gradually decreases from the large diameter portion to the small diameter portion . a plurality of tubes ; a plurality of heads connected to the tubes and configured to accommodate heated steam; a manifold connected to the heads and configured to accommodate heated steam; and a first link pipe and a second link pipe configured to connect the heads and the manifold , characterized in that : the first link pipe or the second link pipe comprises a variable pipe whose inner diameter gradually changes; wherein the first link pipe comprises a first connection link part extending from an associated one of the heads toward the manifold and a first inclined link part protruding obliquely from the first connection link part and coupled to the manifold ; wherein the first inclined link part comprises a large diameter portion having a larger outer diameter than the first connection link part , a small diameter portion having the same inner diameter as the first connection link part , and the variable pipe connecting the large diameter portion and the small diameter portion; and wherein the large diameter portion is connected to the manifold , and the inner diameter of the variable pipe gradually decreases from the large diameter portion to the small diameter portion . the first link pipe or the second link pipe comprises a variable pipe whose inner diameter gradually changes; wherein the first link pipe comprises a first connection link part extending from an associated one of the heads toward the manifold and a first inclined link part protruding obliquely from the first connection link part and coupled to the manifold ; wherein the first inclined link part comprises a large diameter portion having a larger outer diameter than the first connection link part , a small diameter portion having the same inner diameter as the first connection link part , and the variable pipe connecting the large diameter portion and the small diameter portion; and wherein the large diameter portion is connected to the manifold , and the inner diameter of the variable pipe gradually decreases from the large diameter portion to the small diameter portion . <EOS>