The formation diagram of Bi-Fe-OH^- system in the aqueous solution was studied using hydrothermal or "wet" method. Three kinds of bismuth-ferricoxides, Bi₂₅FeO₄₀ (Fe-sillenite), BiFeO₃ and Bi₂Fe₄O₉ were obtained depending on their reaction conditions. The stoichiometric Bi₂₅FeO₄₀ has two kinds of particle shapes, the cubic and the tetrahedral, depend on its reaction conditions, although formation mechanism of the two kinds of particle shapes is not clear yet. The precipitate obtained below 100°C from starting materials with higher ratio of Fe/Bi than 3.8mole% showed interesting feature as follows. The precipitate gave only analogous XRD spectrum of Bi₂₅FeO₄₀, but observations by SEM and Mossbauer effect showed the surface of the precipitate was coated homogenously with fine particles of amorphous Fe oxide. The Bi₂₅FeO₄₀ coated with amorphous fine particles of Fe oxide (hereafter described as pseudo Fe-sillenite) reacted with CO₂ gas or CO₃^2- in aqueous solution and easily converted to Bi₂O₂(CO₃) at the mild reaction conditions. The gradual conversion took place even at the room temperature during the prolonged period. The phenomena may be applied to the immobilization of the CO₂ in the air and in the aqueous solution.