PURPOSE: To control the water level of a hot-water reserving tank correctly and permit to miniaturize the hot-water reserving tank by a method wherein a water level detecting unit for a water level detecting means, interrupting the supply of water from a water supplying source by detecting a water level at a level lower than the water level of the hot-water reserving tank, is arranged in an oveflow tank.

CONSTITUTION: A heat collecting unit 3, heating cool water by the solar heat, the hot- water reserving tank 1, reserving the hot-water heated by said heat collecting unit 3, and the water supplying source, connected to the hot-water reserving tank 1, are arranged in this system. A water supplying port 7, connected to the water supplying source, is opened in the hot-water reserving tank 1 while the overflow tank 4, keeping the water level of the tank 1 in constant, is provided. A float 10 for a ball tap 9, detecting the water level lower than the same in the tank 1 and interrupting water supply from the water supplying source, is arranged in said tank 4, to detect the water level of the tank 4, which is lowered upon supplying the hot-water, by the float 10 and supply the water to the tank 1 from the water supplying source. According to this method, the water level of the tank 1 may be controlled more correctly and the tank 1 may be made compact by decreasing a space above the water level of the tank 1.