PURPOSE: To remove CO2 efficiently in a simple method by contacting the CO2 contained exhaust gas with highly active calcium oxide porous particles having a specified specific surface area and a specified particle size at temperature within a specified range.

CONSTITUTION: Highly active calcium oxide porous particles are obtained which is characterized by the specific surface area of 5-60m2/g or more preferably 10-50m2/g, and the diameter 1-10mm, preferably 3-6mm. In order to come in contact with the porous particles, exhaust gas is passed through an apparatus composed of a cylindrical treatment tube which is filled with the particles. A net or glass wool is placed above and under the particles to hold them. A part of the column which is filled with the particles is heated by an appropriate means such as electric heating, infrared heating, and high-frequency heating to heat the particles at 400-700°C preferably 500-650°C. In this way, the CO2 in exhaust gas can be removed efficiently for a long period of time.