A carbon dioxide concentration-controlling device (1 or 100) of the present invention controls the carbon dioxide absorption rate of a carbon dioxide-absorbing material (30) in accordance with a carbon dioxide concentration detected by a detector (20). A state determining unit (11) determines a state of the carbon dioxide-absorbing material (30), and a state controlling unit (12) controls the state of the carbon dioxide-absorbing material (30) to the state determined by the state determining unit (11).