Utilizing incineration residues such as fly ashes and bottom ashes as a cement resource is a potential alternative to landfill. In recycling fly ashes, in particular, the risk caused by dioxins or heavy metals in the fly ashes should be considered. We have previously found that dioxins in fly ashes are degraded by mechanochemical (MC) treatment with CaO. In this paper, we report the elution properties of heavy metals, Pb, Cd, Se, As and Cr6+, from fly ashes treated mechanochemically with CaO and subjected to cementation. Pb elution from fly ashes was inhibited by 93% by MC treatment as compared to untreated fly ashes, and the cementation of mechanochemically treated fly ashes inhibited elation by 100%. The same effects were confirmed for the other metals. The material of a reactor should be selected appropriately, because the abrasion of the reactor by pulverization influences the elution of a toxic heavy metal, for example, Cr, contained in stainless steel reactor.