NOVELTY - The system has a hydrolyzing unit (3) which is included with a pair of electrode with which the optical semiconductor catalyst is supported by one side. The electrolyte solution is decomposed using the sunlight energy irradiated to optical semiconductor catalyst. Fuel cell (20) generates electric-power using the hydrogen produced by hydrolyzing unit. Generated water supply line (23) supplies generated water produced in the fuel cell to the hydrolyzing unit. Water-consumption calculating unit calculates consumption of water in hydrolyzing unit.    USE - Sunlight utilization system for fuel cell reaction.    ADVANTAGE - By supplying generated water to hydrolyzing unit, the reduction of the electrolyte solution in water-electrolysis unit is suppressed, and the maintenance is made easy. Since the quantity of the water which should be added to hydrolyzing unit from the outside calculates automatically, the maintenance can be made easy. Since the abnormal circumstances are perceived automatically and abnormal signal is output, the operator can cope with abnormal circumstances quickly. The moisture-content deterioration of the electrolyte accompanying electrolysis of water in the hydrolyzing apparatus can be replenished.    DETAILED DESCRIPTION - Generated water supply-amount calculating unit calculates quantity of the generated water supplied to the hydrolyzing unit from the fuel cell. Additional water amount calculating unit calculates water quantity which compares the water consumption and the generated water supply-amount, and adds to the hydrolyzing unit from the outside. Water quantity detection unit detects real water quantity of hydrolyzing unit. Real water quantity comparison unit compares the detected real water quantity with a predetermined threshold value. Alarm unit outputs an abnormal signal, when detected real water quantity deviates from the predetermined threshold value. Hydrolyzing unit includes a circulation line used to circulate electrolyte. The generated water of the fuel cell is introduced to circulation line.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the sunlight utilization system. (Drawing includes non-English language text)    Hydrolyzing unit (3)    Fuel cell (20)    Generated water supply line (23)    Pump (25)    Additional water amount control unit (40)