A new type of carbon dioxide (CO₂) removal substance so called solid amine has good features to keep a closed cabin atmosphere clean in manned spacecraft. It can adsorb rare CO₂ gas repeatedly by use of regeneration process with small amounts of thermal energy and/ or depressurizing. It is also available to adsorb water composition simultaneously. An ordinary CO₂ removal agent, lithium hydroxide (LiOH), has larger amounts of CO₂ removal capacity than that of solid amine; however, decomposition process to purge reacted CO₂ requires high temperature and large amounts of energy, so that loading weight will become larger in proportion as duration time and crew number. Another well-known CO₂ removal agent, zeolite molecular sieves, needs a desiccant process to efficiently adsorb CO₂ gas. Solid amine has unique characteristics in comparison with lithium hydroxide and zeolite. Basic characteristics of solid amine are described in another paper (12) concerning adsorption equilibrium, breakthrough data in a fixed bed system, and diffusion mechanism under N₂ gas atmosphere condition. This paper deals with moisture adsorption on solid amine, effects of humidity to CO₂ removal capacity, and mechanism of bicomponent adsorption of CO₂ and moisture in air at room temperature.