PURPOSE: To contrive an increase of a quantity of heat transmission per pipe without heating a return part of a condensed liquid, by making the inside of a descending pipe of the inside of an outer pipe into structure wherein the condensed liquid flows.

CONSTITUTION: An outer pipe 1 consists of mainly a heat receiving part 7 and a condensation part 6. As the heat receiving part 7 is heated in the inside of a high heat source, a working fluid 5 in the inside boils, gaseous phase and a liquid phase are separated by a vapor-liquid separator 3 and the major part of the gaseous phase is arrived at the condensation part 6. On the one hand, as heat of the condensation part 6 is radiated in the inside of a low heat source, a gaseous phase ingredient out of the working fluid 5 is condensed here, and heat transmission is made to the low heat source. A liquid phase ingredient separated by the vapor-liquid separator 3 and the liquid phase ingredient condensed by the condensation part 6 are descended within an inner pipe 2 and returned to the heat receiving part 7 through an open end of the lower part of the inner pipe 2. The heat transmission from the high heat source to the low heat source is made in this manner. As the major part of a condensed liquid is descended along the inner pipe 2, spontaneous circulation of the working liquid can be realized as compared with a heat pipe and sufficient supply of the condensed liquid to the heat receiving part is done.