NOVELTY - A three-chamber-type electrolytic cell comprises an anode chamber (3), an intermediate chamber (2) which is poured with water containing tritium and hydride and separated from anode chamber by anode side electrolyte layer (6), and cathode chamber (1) separated from intermediate chamber by cathode side electrolyte layer (7). Light energy having wavelength more than wavelength at which energy exceeding oxygen-hydrogen bond energy is given and bonding is decomposed, and less than wavelength in which bonding of hydride is decomposed, is irradiated to intermediate chamber.    USE - Three-chamber-type electrolytic cell is used for tritium collecting system (claimed).    ADVANTAGE - The three-chamber type electrolytic cell improves the tritium removal rate, and provides tritium collecting system which enables efficient recovery of tritium with less light energy.    DETAILED DESCRIPTION - A three-chamber-type electrolytic cell comprises an anode chamber, in which an anode (8) is placed, an intermediate chamber which is poured with water containing tritium and hydride and separated from the anode chamber by anode side electrolyte layer, and a cathode chamber in which cathode (5) is placed and separated from the intermediate chamber by cathode side electrolyte layer. The bond energy of hydrogen of the hydride and another element is less than oxygen-hydrogen bond energy of oxygen and hydrogen in water. Light energy having wavelength more than the wavelength in which the energy exceeding oxygen-hydrogen bond energy is given and bonding is decomposed, and less than the wavelength in which the bonding of the hydride is decomposed, is irradiated to the intermediate chamber. An INDEPENDENT CLAIM is included for tritium collecting system, which comprises multistep structure in which the treated water discharged from the three-chamber-type electrolytic cell flows in into the three-chamber-type electrolytic cell of next process.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of three-chamber type electrolytic cell.    Cathode chamber (1)    Intermediate chamber (2)    Anode chamber (3)    Cathode (5)    Anode side electrolyte layer (6)    Cathode side electrolyte layer (7)    Anode (8)