PURPOSE: To drastically increase the reaction time of a chemical bed and to accurately detect the exchange time of the chemical bed by providing a first bed in which the necessary amt. of the main chemical and the necessary amt. of a neutralizer minus a specified amt. and a second bed of only the specified amt. of neutralizer arranged in the succeeding stage.

CONSTITUTION: This device consists of a first bed 3 in which a necessary amt. of a main chemical 1 for absorbing the main component in a waste gas except the acidic component and a necessary amt. of a neutralizer 2 for absorbing the acidic component minus a specified amt. are mixed and a second bed of only the specified amt. of the neutralizer arranged in the succeeding stage. Consequently, the temp. of the second bed 4 is increased above that of the first bed 3 by the exothermic reaction between the neutralizer and the acidic component, and the temp. exceeds the preset value about the time when the neutralization reaction is completed. Accordingly, provided the fact that the temp. of the second bed 4 exceeds the preset value is detected, the exchange time of the chemical bed is immediately and accurately detected.