PURPOSE: To effectively utilize thermal energy and to eliminate the insufficient heat of new water in winter or in cloudy day by applying thermal energy of hot exhaust water newly to new water to be filled in a solar heat collector.

CONSTITUTION: The interior of a two-room heat sotrage hot water tank 1 with a heat exchanger is divided by a partition plate 2 into a new water tank 3 and an old water (hot exhaust water) tank 4. The upper end of the tank 3 communicates with the outlet of a solar heat collector 7 through a pipe 6, and the lower end communicates with the inlet of the collector 7 through a pipe 8. Further, the other ends of a pipe 9 which communicates with the lower end of the tank 3 and a pipe 10 which communicates with the lower end of the tank 4 are connected to a three-way valve 11, which is connected to the lower end of a bathtub 15 of a bath through a pipe 14 having a pump 12 and a valve 13. One end of the pipe 14 is connected to an exhaust port through a drain valve 16, and the other has at the end a cock 17. Since the collector 7 which circulates and heats new water which is heated in advance is connected to the tank 1, the thermal energy of the hot exhaust water can be effectively utilized as the energy for heating the new water in advance, thereby eliminating the insufficient heat of the new water in winter or in cloudy day.