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In order to use waste materials effectively, a possibility of simultaneous solidification of inorganic waste materials and fixation of alkaline ions and heavy metal through formation of zeolite by using hydrothermal processing, was investigated. Fly ash, cinder of sewage sludge, and cinder of rubbish were used as the starting materials. Ca/Si and 2Na/Si molar ratios of the specimens were changed by the addition of CaO and Na(sub 2)CO(sub 3). These mixtures were formed and hydrothermally processed at 200 degrees C. Various zeolites, such as analcime, Linde B(sub 1) zeolite, cancrinite, zeolite P-C, and hauyne, were formed at Ca/Si = 0.3 or 0.5 and 2Na/Si = 0.1 - 1.0. In the case of the fly ash system, although only the zeolite minerals were formed, the flexural strength reached up to 17 MPa when Ca/Si = 0.3 and 2Na/Si = 0.1, and the fixing ratio of Na(super +) ion reached a maximum value of 85% by the formation of zeolites. When cinder of rubbish was used, dissolution of heavy metals from the solidified specimen was scarcely observed. The microstructure and the variation of pore size distribution were also discussed.