PURPOSE: To prevent the liquid film of working fluid from breaking or dry patches from developing and consequently obtain very favorable heat transfer characteristics even when the supply of working fluid is scarce by a structure wherein at least one cup-shaped liquid receiver is mounted on the outer surface of each vertically installed heat transfer pipe.

CONSTITUTION: The fed working fluid L flows from a distributing plate 3 along the outside of each heat transfer pipe 2 in the form of liquid film 10 and, while flowing down, heated by the hot water flowing in the pipe 2 and evaporated. Because cup-shaped liquid receivers 13 are mounted on the outside of the heat transfer pipe 2 at proper intervals, the working fluid flowed down in liquid film once pools in the receiver 13 and, after that, overflows so as to flow down in liquid film again. In an evaporator constituted as just mentioned above, because the cup-shaped liquid receiver 13 acts as the distributor to flow the working fluid uniformly, the breakage of liquid film or the development of dry patches over the total length of the heat transfer pipe can be prevented from occurring even when the supply or working fluid is scarce, resulting in enabling to keep the heat transfer characteristics original to the heat transfer pipe.