To provide a carbon dioxide adsorbent which can recover CO2 efficiently from a wet gas even in a high-temperature atmosphere of ≥80°C.

This carbon dioxide adsorbent (the adsorbent I or the adsorbent II) is composed of a porous substance having a minute void and the object to be deposited in the minute void. The object to be deposited contains one or both of rubidium carbonate and cesium carbonate. The object to be deposited can contain potassium carbonate. The porous substance consists of one of activated carbon, zeolite, diatomaceous earth and alumina or a mixture of any two or more of them.