PURPOSE: To achieve a decrease in cost and to effectively utilize a lightweight concrete powder by bringing exhaust gas into contact with calcium silicate containing tobermorite to remove CO2, SOM and NOM in the exhaust gas and preparing a silica gel by the contact with the exhaust gas.

CONSTITUTION: Exhaust gas is brought into contact with calcium silicate containing tobermorite (represented by 5CaO.6SiO2.5H2O usually contained in a powder or granule of lightweight concrete for example, cured in an autoclave in an amount of 80wt.%) to remove CO2 and/or SOM and/or NOM in the exhaust gas. When the exhaust gas is brought into contact with tobermorite- containing calcium silicate, CaO in calcium silicate is compounded with carbon dioxide, sulfur dioxide and nitrogen oxide in the exhaust gas to elute as calcium carbonate, gypsum and calcium nitrate and, therefore, a silica gel having a tobermorite skeletal left therein is obtained. By this method, cost is not raised and there is no problem treating a byproduct and a lightweight concrete powder can be effectively utilized.