PURPOSE: To obtain a heating apparatus with simple structure which needs no power for moving a working fluid by circulating the working fluid cyclically through stages for evaporation-diffusion in the gaseous phase-condensation-movement in the liq. phase in a vacuum vertical hollow metallic body.

CONSTITUTION: A hollow radiator 1 is manufactured with two sheets of light metal plates, a pot 2 of the same metal for heating the working fluid is formed at a right end of the bottom, the inside is brought to vacuum or approximately vacuum, the working fluid such as water is sealed therein, and the bottom is tilted toward the pot 2. According to this constitution, the working fluid is heated through a heater 5 and a thermal medium 3, and evaporated. Vapor is diffused into the hollow body in vacuum, contacts with a radiation surface 1, condenses, and radiates heat, thus displaying a heating effect. The condensed working fluid is changed into drops, falls in grooves 6 by gravity, and returns into the pot 2. Thus, the heating apparatus requiring no power for moving the working fluid can be obtained by the simple structure.