Synthesis and CO₂ adsorption ability of calcium-deficient tobermorite were investigated. The calcium-deficient tobermorites with Ca/(Al+Si) atomic ratios lower than that of the theoretical composition, 0.83, were prepared by the reaction in the system CaCl₂-Na₂SiO₃-AlCl₃-H₂O under atmospheric pressure. The calcium-deficient tobermorites were characterized by means of X-ray diffraction, thermal analysis (TG-DTA), infrared spectroscopy and chemical analysis. The calcium-deficient tobermorite was formed from starting solutions with Ca/(Al+Si) atomic ratios lower than 0.95. Most Ca²⁺ ions existing in interlayers of the tobermorite, 20% of total Ca²⁺, were removed successively by decreasing the initial Ca/(Al+Si) atomic ratio from 0.95 to 0.70, and then the basal spacing of the calcium-deficient tobermorite expanded by replacing one Ca²⁺ ion site with two Na⁺ ions until the atomic ratio of 0.7. However, the layer structure of the tobermorite was destroyed at the atomic ratios below 0.7. The amount of Na⁺ ions to compensate for charge deficiency in calcium-deficient tobermorites obtained at an initial Ca/(Al+Si) atomic ratio of 0.7 and initial Al/(Al+Si) atomic ratio of 0.10 was changeable by number of washings with pure water. The composition after washing was expressed by the formulas of Ca₄Na_0.4[(Al+Si)₆O₁₁H₂]⋅4H₂O including ion defects after ten times washings (5g/dm³ H₂O) and Ca₄Na_2.1[(Al+Si)₆O₂₂H₂]⋅4H₂O after one time washing (5g/100cm³ H₂O). The maximum exchange capacity for Na⁺→K⁺ of Na⁺ present in the calcium-deficient tobermorite was 95meq/100g irrespective of the amount of Na⁺. The adsorption capacity of CO₂ at 60°C in the tobermorite increased with increasing Na/(Al+Si) atomic ratio and reached a maximum value of 3.0mmol/g at the atomic ratio of 0.36. The desorption of CO₂ and adsorption of Na⁺ progressed simultaneously on washing the tobermorite which adsorbed CO₂ with 1N NaCl solution. Accordingly the tobermorite washed with NaCl solution was available as CO₂ adsorbent. when the adsorption and desorption were repeated many times, the formation of CaCO₃ originated from partial carbonation in the tobermorite was unavoidable, and consequently, the adsorption capacity of CO₂ decreased successively with increasing recycle number. On the other hand, when the calcium-deficient tobermorite after desorption of CO₂ was heated at 200°C, its adsorption capacity was kept at 2.0mmol/g even after using five times.