A method for removing CO2 and H2S from a feed gas including at least CO2 and H2S, comprising: a step of introducing the feed gas into a first absorber, and removing CO2 and H2S from the feed gas by contacting the feed gas with a lean solution in the first absorber; and a step of subsequently introducing the feed gas into a second absorber, and further removing CO2 and H2S remaining in the feed gas by contacting the feed gas with a semi-lean solution in the second absorber; wherein a rich solution composed of the lean solution and the semi-lean solution that have absorbed CO2 and H 2 S in the first absorber and in the second absorber respectively is introduced into a flash drum in which an internal pressure is kept lower than that in the first absorber and in the second absorber; after a part of CO2 and H2S is released from the rich solution and a semi-lean solution is obtained in the flash drum, a part of the semi-lean solution in the flash drum is cooled and introduced into the second absorber to contact again with the feed gas in the second absorber; a remaining part of the semi-lean solution in the flash drum is introduced into a stripper and heated to release remaining CO2 and H2S from the rich solution and a lean solution is obtained in the stripper; a part of the lean solution in the stripper is cooled and introduced into the first absorber to contact again with the feed gas in the first absorber; and after a remaining part of the lean solution in the stripper is cooled, the remaining part of the lean solution is introduced into the second absorber from a upper level than a level of where the semi-lean solution from which a part of CO2 and H2S has been released in the flash drum is introduced, and again is contacted with the feed gas in the second absorber, and thereby obtaining treated gas. a step of introducing the feed gas into a first absorber, and removing CO2 and H2S from the feed gas by contacting the feed gas with a lean solution in the first absorber; and a step of subsequently introducing the feed gas into a second absorber, and further removing CO2 and H2S remaining in the feed gas by contacting the feed gas with a semi-lean solution in the second absorber; wherein a rich solution composed of the lean solution and the semi-lean solution that have absorbed CO2 and H 2 S in the first absorber and in the second absorber respectively is introduced into a flash drum in which an internal pressure is kept lower than that in the first absorber and in the second absorber; after a part of CO2 and H2S is released from the rich solution and a semi-lean solution is obtained in the flash drum, a part of the semi-lean solution in the flash drum is cooled and introduced into the second absorber to contact again with the feed gas in the second absorber; a remaining part of the semi-lean solution in the flash drum is introduced into a stripper and heated to release remaining CO2 and H2S from the rich solution and a lean solution is obtained in the stripper; a part of the lean solution in the stripper is cooled and introduced into the first absorber to contact again with the feed gas in the first absorber; and after a remaining part of the lean solution in the stripper is cooled, the remaining part of the lean solution is introduced into the second absorber from a upper level than a level of where the semi-lean solution from which a part of CO2 and H2S has been released in the flash drum is introduced, and again is contacted with the feed gas in the second absorber, and thereby obtaining treated gas.