In one embodiment, a carbon dioxide capturing system includes an absorption tower configured to bring a treatment target gas containing carbon dioxide into contact with an absorption liquid, and to discharge the absorption liquid having absorbed the carbon dioxide. The system further includes a regeneration tower configured to make the absorption liquid discharged from the absorption tower dissipate the carbon dioxide, and to discharge the absorption liquid having dissipated the carbon dioxide. The system further includes a treatment target gas line configured to introduce the treatment target gas into the absorption tower, a first introduction module configured to introduce a first gas having a higher carbon dioxide concentration than the treatment target gas into the treatment target gas line, and a second introduction module configured to introduce a second gas having a lower carbon dioxide concentration than the treatment target gas into the treatment target gas line.