NOVELTY - The system (1) has an artificial-photosynthesis apparatus (10) which includes a catalyst which receives sunlight and decomposes water. A heat exchanger apparatus (20) performs heat-exchanging between the artificial-photosynthesis apparatuses. A thermal accumulator (30) stores a heat by pooling a thermal medium. A thermal-medium circulation system (40) transmits the heat by circulating the thermal medium between the heat exchanger apparatus and the thermal accumulator.    USE - Solar-energy utilization system.    ADVANTAGE - The temperature of the artificial-photosynthesis apparatus is adjusted by circulating the thermal medium between the heat exchanger apparatus and the thermal accumulator, so that the conversion efficiency of solar energy is improved. The reaction rate of catalyst adjusts the temperature of the artificial-photosynthesis apparatus to the temperature range which becomes high, and the heat of the artificial-photosynthesis apparatus is stored in the thermal accumulator, so that the conversion efficiency of the solar energy is improved more by using the heat for the arbitrary objective.    DETAILED DESCRIPTION - A primary temperature sensor (50) detects the temperature of artificial-photosynthesis apparatus. A secondary temperature sensor (60) detects the temperature of thermal accumulator. The temperature of artificial photosynthesis apparatus is less than one target temperature. A control apparatus (70) performs control which transmits the heat of thermal accumulator to artificial-photosynthesis apparatus by circulating the thermal medium between the heat exchanger apparatus and the thermal accumulator, when temperature of artificial-photosynthesis apparatus is less than temperature of thermal accumulator. The control apparatus performs control which transmits the heat of artificial-photosynthesis apparatus to the thermal accumulator by circulating the thermal medium between the heat exchanger apparatus and the thermal accumulator with the thermal-medium circulation system when the temperature of the artificial-photosynthesis apparatus is higher than the temperature of the thermal accumulator. The control apparatus computes the required calorie amount required when the temperature of artificial-photosynthesis apparatus turns into one target temperature and when the artificial-photosynthesis apparatus does not receive sunlight. The control apparatus computes duration required to transmit the required calorie amount to the artificial-photosynthesis apparatus from the thermal accumulator, and performs control which transmits the heat of thermal accumulator to artificial-photosynthesis apparatus, only when the time by the time estimated to enable the artificial-photosynthesis apparatus to receive sunlight from present time turns into below the duration. The temperature of the thermal accumulator is used as a value of one target temperature when the one target temperature is higher than the temperature of the thermal accumulator.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the solar-energy utilization system.    Solar-energy utilization system (1)    Artificial-photosynthesis apparatus (10)    Heat exchanger apparatus (20)    Thermal accumulator (30)    Thermal-medium circulation system (40)    Primary temperature sensor (50)    Secondary temperature sensor (60)    Control apparatus (70)