We prepared a new glass ionomer cement using bioactive CaO-P₂O₅SiO₂(-MgO) glass and investigated its setting process using FT-IR and MAS NMR analyses. The compressive strengths of the cements depended on the glass composition and a maximum strength of 33.3±4.7MPa was obtained using cement with the glass composition of MgO: 4.6, CaO: 44.9, SiO₂: 34.2 and P₂O₅: 16.3% in weight. FT-IR analysis showed that the COOH group in the polyacrylic acid decreased and carboxylate ion (COO^-Ca²⁺) increased after the setting reaction. A broad signal appeared around -82ppm in ²⁹Si MAS-NMR spectra of the glass and a new signal corresponding to hydrated silica gel formation appeared around -102 and -111ppm after setting. This suggests that Ca²⁺ was released from the glass powder to form carboxylate salt and that a degree of polymerization in the silicate network increased. The setting mechanism of the cement was found to be essentially the same as in conventional glass ionomer cement.