PURPOSE: To raise heat transfer efficiency by providing a pipe passage which connects the upper header of a group of evaporation pipes and a second condensation pipe in a low temperature gas duct and separating the gas which was not condensated by arranging the direction of the vapor flow of a working medium and the direction of the flow of a condensation liquid mutually oppositely.

CONSTITUTION: The water in a group 1 of evaporation pipes becomes steam by a high temperature gas and condenses in a group 3 of condensation pipes in a low temperature duct 4 via an upper header 1, pipe passage 5, and upper header 3b and discharges the condensation latent heat to the low temperature gas and returns to the group 1 of evaporation pipes via a lower header 3a, pipe passage 6, and lower header 1a. On the other hand, part of the steam condenses while it rises in a heat pipe 12 which is a second condensation pipe via a pipe passage 10, and lower header 12a, and the condensing water falls by gravity and enters the pipe passage 6 and incondensible gases are effectively separated from the steam flow by the rising air flow and the downward flow of the water mutually in opposite directions and collect in the upper header 12b, and they are discharged through a gas storage 17 by opening and closing of valves 16a and 16b. It is, therefore, possible to separate and discharge incondensible gases effectively and improve thereby efficiency in heat transfer.