To provide a heat utilization system capable of effectively utilizing solar heat and geothermal heat regardless of seasons and a surrounding environment.

The system comprises a hot-water supply tank 20 for storing high-temperature hot water, a heat storage tank 30 for storing hot water at temperature lower than that of the hot-water supply tank 20, heat exchanging units 28, 38 provided for each water storage tank, a solar heat collecting panel 10, a heat storage part 60 with its main part laid under the ground, a medium circulation route 40 for circulating a heat exchanging medium selectively in the solar heat collecting panel 10 and the heat storage part 60, a two-way valve 44 for controlling whether the heat exchanging medium circulating in the medium circulation route 40 is made to circulate in the solar heat collecting panel or not, three-way valves 42, 43 for controlling whether the heat exchanging medium is made to circulate in the respective heat exchanging units or not, valves 44, 47, 49 for controlling whether the heat exchanging medium is made to circulate in the heat storage part 60 or not, and a control part 100 for controlling the control valves. The control part 100 controls the respective control valves based on temperature of the heat exchanging medium.