PURPOSE: To prevent a leaking gas stream, by a method wherein cages provided in a multistage are filled with an adsorbent and gaps formed by a cage support, the inner wall of an adsorbing tower and the cage are also filled with the adsorbent.

CONSTITUTION: Doors 12a, 12b are opened to set cages 9a, 9b filled with an adsorbent on supports 11, 11' respectively and the door 12a is closed to fill the gap between the cage 9a and a tower body 1 and the gap between the cage 9a and the door 12a with the separately prepared adsorbent 3 and the door 12b is closed to similarily fill the gap with the cage 9b with the adsorbent 3 and a door 12c is closed at last. When a gas to be adsorbed is introduced from an inlet 4, said gas is entirely passed through the adsorbent. When the supports 11, 11' are attached obliquely, the adsorbent is dropped from the gap when the door 12b is slightly opened in taking out the cage to be received by the cage and, when the cage 9b is detached, the adsorbent is dropped onto the next lower stage and, about the cage at the lowermost stage, because the adsorbent filling the gap is dropped onto a manhole 8, it may be taken out by opening the manhole 8.