PURPOSE: To easily control an adsorbent and to improve the reliability of an adsorption tower by additionally providing a heater to regenerate the adsorption tower, if necessary, at high temp. in addition to the ordinary regeneration of the tower at low temp.

CONSTITUTION: In adsorption towers 1a and 1b for removing CO2, a heater 6 is used to regenerate an adsorbent (e.g. molecular sieve) at low temp. in normal operation, and a regenerating heater 7 is used when the adsorptivity is lowered. Namely, even when the adsorbent is humidified in maintenance, etc., and not regenerated by low-temp. regeneration, the adsorbent is regenerated at high temp., the adsorptivity to CO2 is secured as planned, the reliability of the adsorption tower is secured, the maintenance of the adsorbent is not limited, and the maintenability is improved.