PURPOSE: To improve the properties of installation and safety by a constitution wherein operating liquid is evaporated in an evaporating unit and is sent to a condensing unit to condense and reserve in a liquid reservoir in the day time while the operating liquid is evaporated and is condensed in the evaporating unit when the sun is not shining.

CONSTITUTION: A part of a pipeline 2e connecting the evaporating unit 2a to the condensing unit 2c is flexible. In the day time and the sun is shining, the operating liquid 5 in the evaporating unit 2a of a heat pipe 2 for a heat collector 1 is heated to a gaseous condition, moved to a condensing unit 2c, condensed and liquefied, thereafter, is reserved in the liquid reservoir 2d. At night or when the sun shine is weak in the day time, the temperature of the heat collector 1 is reduced, the internal pressure of the evaporating unit 2a becomes lower than the same of the liquid reservoir 2d, the operating liquid 5 in the liquid reservoir 2d is evaporated and is moved to the evaporating unit 2a, then, is condensed and is reserved in the liquid reservoir 2b. Restriction of positional relation between the evaporating unit 2a and the condensing unit 2c is not generated, therefore, the heat collector 1 is installed on the surface of a roof at a ridge side while a hot-water reserving tank 3 is installed on the other side, thus, sliding and dropping of the device may be prevented.