A gas treatment device uses a gas to be treated which contains an acid compound that dissolves into water to produce acid and a treatment liquid which absorbs the acid compound to phase-separate, to separate an acid compound from the gas to be treated. The gas treatment device includes an absorber which brings the gas to be treated into contact with the treatment liquid; a regenerator 14 which heats the treatment liquid contacting the gas to be treated to separate an acid compound; and a liquid feeding portion which feeds the treatment liquid contacting the gas to be treated in the absorber to the regenerator. In the absorber, the treatment liquid contacting the gas to be treated phase-separates into a first phase portion having a high acid compound content and a second phase portion having a low acid compound content. The liquid feeding portion is configured to introduce, into the regenerator, the treatment liquid with the phase-separated first phase portion and second phase portion mixed.