An absorbing solution according to the present invention is an absorbing solution that absorbs CO2 or H2S in gas or both of CO2 and H2S. The absorbing solution is formed by adding desirably 1 to 20 weight percent of tertiary monoamine to a secondary-amine composite absorbent such as a mixture of secondary monoamine and secondary diamine. Consequently, it is possible to control degradation in absorbing solution amine due to oxygen or the like in gas. As a result, it is possible to realize a reduction in an absorption loss, prevention of malfunction, and a reduction in cost. This absorbing solution is suitably used in an apparatus for removing CO2 or H2S or both of CO2 and H2S.