PURPOSE: To make it possible to manufacture very slender, long heat pipes by constituting a heat pipe into loop shape and circulating in the same direction working liquid in gas phase end working liquid in liquid phase and conveying under pressure in the direction of the circulation the closed working liquid by the pressure of working liquid vapor which is developed at an evaporation section.

CONSTITUTION: A container of loop shaped heat pipes is constituted with a loop 1-2-3-4 of small tube containers 1 and 3 which have capillary action and working liquid storage small tube containers 2 and 4 connected in series. The inside of the working liquid storage small tube container is provided with circulation direction regulating means 5 and 6 so that the directions of flow of the working liquid and the working liquid vapor are controlled. When there is a temperature difference between evaporation sections A and B, the working liquid moves in the small tube containers 1 and 3 in the direction of the arrow mark by the difference of the internal pressures and by the aid of the non-return action of working liquid circulation direction regulating means. When the internal pressures of the containers are substantially uniform, the small pipe containers 1 and 3 supply the working liquid to the evaporation section by the capillary action. The action of the working liquid circulation direction regulating means circulates the working liquid in a constant direction and prevents the back-flow of the working liquid.