To increase absorptivity of an adsorbent by introducing the flame of the burner into the furnace to carbonize the carbonizing material, as the gas in the oven is sucked with an ejector arranged beneath the carbonizing oven and introduced into the flue gas duct.

The cover 12 for the main body 1 of the carbonizing oven is opened, a metallic cage 2 containing the material to be carbonized is set in the oven and the cover 12 is set and the oven is tightly closed. Then, the sucking turbo fan 6 is driven to suck the gas in the oven into the flue gas duct 11 arranged over the valve 7 for taking out wood vinegar. Then, the burner 3 is driven, the flame is introduced through the hot air duct 4 into the furnace 12 to carbonize the material to be carbonized. This carbonized product is crushed, combined with a naturally occurring zeolite powder and a binder, kneaded and formed to give the objective adsorbent.