To construct the ITER with high safety and acceptability, it is very important to grasp the removal behavior of tritium happened to leak in the room, the final confinement barrier. In order to obtain data on tritium removal behavior from atmosphere in a room under the various conditions (humidity, ventilation flow rate), intentional tritium release experiments have been carried out with the Caisson Assembly for Tritium Safety Study (CATS) which consists of 12 m3 gas-tight box (Caisson) for the study of tritium behavior in large space. Effect of adding water vapor has also investigated for effective removal. When the tritiated water existed in the released tritium, residual contamination on the wall of the Caisson was detected under the various ventilation flow rate and it was found that it depended on the initial humidity in the Caisson. On the other hand, when the water vapor was added into the Caisson after found the residual contamination, the residual contamination was removed quickly on the wall of the Caisson. The adding water vapor into the Caisson, it was effective for the tritium removal. Analytical work have also progressed and analyzed tritium removal behavior became to be in good agreement with the experimental results by considering the adsorption and desorption reaction rate of tritiated water on the wall.