The solidification/stabilization mechanism of each cementious material was investigated. It was found that when $C_3$S was hydrated , the Pb element could be transferred to the insoluble Ca[Pb(OH)$_3$.$H_2O$]$_2$and the Cr element to the CaCr $O_4$$H_2O$. The addition of heavy metal tends to delay the hydration until initial 7 days. The Pb element as also delayed the hydration and the Cr element was substituted for the ettringite. On the occasion of the hydration of $C_4$ $A_3$ $S^{S}$, the Pb and Cr ions were solidified/stabilized by the substitution into the ettringite and/or monosulfate. Leaching of the Pb, Cr and Zn elements in the solidified material was extremely little, indicating that heavy metals were effectively solidified/stabilized in the hydrated cementious materials. Solidification/stabilization of heavy metal ions in the industrial wastes such as the STS, BF and COREX sludge was investigated. In case of the mixing ratio of cement and slag was 3 : 7, leaching of hazardous heavy metal ions was very little, indications that the solidification and stabilization was very successful.l.