PURPOSE: To utilize the whole surface of a plate for heat transfer effectively and improve heat transfer performance, by a method wherein a pair of flat sheet type plates is welded air-tightly with each other along the peripheral rims to form a sealed space therein and operating fluid is encapsulated into the sealed space.

CONSTITUTION: A heat plate P is formed by welding two sheets of plates P1, P2 air- tightly with each other along the peripheral rims 2 and a sealed space 4, for encapsulating operating fluid, is formed therein. The lower part of the sealed space 4 serves as an evaporating section 6, in which the operating fluid deprives heat from external atmosphere and evaporates, while the upper part thereof serves as a condensing section 8, in which the produced vapor of the operating fluid releases heat to the external atmosphere and is condensed. The condensed operating fluid flows down along the inside surfaces of the plates P1, P2 and circulates to the evaporating section 6. The sealed space 4, located between the evaporating section 6 and the condensing section 8, serves as the circulating section of the operating fluid in such a manner; however, this space 4 serves not only to help the circulation but also contributes to effective heat transfer.