PURPOSE: To conduct heat transmission from the upper part to the lower part, by a method wherein a liquid reservoir is provided at the place upper than an evaporating part within en route of a return passage which introduces condensed fluid from a condensing part while a check valve is provided on a return passage between the reservoir and the evaporating part.

CONSTITUTION: In the daytime, heat medium 4 becomes steam by the sun applied to the evaporating part 1. Steam enters the lower condensing party through an outward passage 10. The heat of steam is transmitted still more from water 7 to a cold water tank 9. Water in said tank 9 becomes hot water by absorbing heat, and is used in a building 2. Condensed fluid is moved under pressure so as to rise a returning passage 11. While the pressure of steam at the evaporating part 1 is high, the check valve 13 closes the return passage 11 in the manner that the steam does not flow reverse towards the reservoir 12. Accordingly, condensed fluid is storaged in the reservoir 12. While at night, the evaporation of heat medium becoming very small, the pressure of steam at the evaporating part 1 gets low, and the check valve 13 is opened, and condensed fluid in the reservoir 12 drops into the evaporating part 1 under gravity.