PURPOSE: To reduce a heat loss due to evaporation and increase the heat accumulating effect of the titled device by a method wherein cylindrical glass tubes, of which insides are evacuated, are floated on the surface of a liquid closely to insulate the surface of the liquid without spoiling the liquid surface and decrease the free surface area of the liquid.

CONSTITUTION: In a non-salty or salty solar pond 1, multitude of cylindrical glass tubes 4W6, having evacuated insides and circular or elliptic sections, are floated under a condition that they are arranged closely each other and floated on the level 3 of a heat accumulating liquid 2. Solar beams 10, 11 pass through the glass tubes 4W6 and heat the heat accumulating liquid 2, however, the interiors of the glass tubes 4W6 are evacuated, therefore, the insulating property thereof toward upward is excellent. Further, said tubes are arranged on the surface of the liquid, therefore, the evaporating surface of the liquid may be constant and sufficiently small in spite of the up-and-down motion of the liquid surface. Accordingly, the temperature obtained by the heat accumulating liquid 3 may be kept for a sufficient long period of time.