PURPOSE: To facilitate manufacturing without increase in weight to obtain heat receiving and transferring area as desired by shaping the closed container like a flat plate.

CONSTITUTION: A closed container 21 is formed like a flat box by combining flat top plate 22 and bottom plate 23 and four side plates. A plurality of wicks each molded like a pipe are arranged in parallel from an evaporating section 26 to the condensating section 27 in the container 21. In operation, steam, as a working medium 28 generated at the evaporating section 26, moves to the condensating section 27 passing between the wicks 25 and 25. A medium 31 to be heated is heated by repeating the evaporation/condensation cycle of the working medium 28. Therefore, it is possible to obtain the desired heat receiving the transferring areas without increase in the weight thereby facilitating the manufacturing of the container.