A once-through heat exchanger comprising: a plurality of tubes (166, 167); a plurality of heads connected to the tubes (166, 167) 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 (162ab, 163ab) 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 (166, 167); a plurality of heads connected to the tubes (166, 167) 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 (162ab, 163ab) 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 (162ab, 163ab) 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.