[en] Purpose: To obtain a high coefficient of separation at a single concentration stage and to provide readiness in maintenance and handling of apparatus. Constitution: Treated water introduced from a treating liquid tank to a water pipe by a pump is pressed by gravity within the water pipe, moves upwardly within an adsorption pipe filled with dried adsorbent (silica gel or other water absorptive adsorbent), and flows out through the valve. When HDO and HTO concentration in the outflowing water becomes equal to the treated concentration, pump is stopped and valve is opened to return the water in the adsorption pipe to the tank. Next, the valve is closed and the adsorption pipe is heated by a heater to expel the adsorbed water. Since the initial outflow portion is low in concentration of heavy water, it is returned to the tank. The successively flowing out portion of high concentration in heavy water is fed into the treated liquid tank on the side of high concentration for further concentration. (Yoshino, Y.)