This air composition adjusting device is provided with: carbon dioxide separation units (41, 61) which separate carbon dioxide from air to be treated that is to be supplied to a target space (5)； a gas supply path (135) which includes a high concentration gas supply path (136) and connects from the carbon dioxide separation units (41, 61) to the target space (5)； and a controller (110) which performs an operation to increase carbon dioxide concentration, in which high carbon dioxide concentration gas having a higher carbon dioxide concentration than the air prior to treatment is supplied to the target space (5) through the high concentration gas supply path (136).
