PURPOSE: To drain water in a reliable manner, by feeding water to a heat collecting panel from a cistern tank installed at a level lower than that of the heat collecting panel, and feeding hot water to an upper space of a heat accumulating tank having the level of water surface at a position higher than the lowermost part of the heat collecting panel.

CONSTITUTION: A cistern tank 9 is installed at a level lower than the lowermost level 19 of a heat collecting panel 1. Water is supplied to the lower end portion of the heat collecting panel 1 by a pump 15, and hot water is supplied from the upper end portion of the heat collecting panel 1 via a pipe 17 to an upper space of a heat accumulating tank 3 having the level of water surface 8 at a higher position than the level 19. A lower portion of the heat accumulating tank 3 is communicated with the inside of the cistern tank 9 via a pipe 11 and a ball tap 12. With such an arrangement, water in the panel 1 is drained into the tank 9 via a pipe 16 and the pump 15.