Cement solidification/stabilization is an effective way of suppressing the leaching of hazardous materials from municipal solid waste incinerator (MSWI) fly ash, though the solidification of the fly ash from fluidized bed incinerators has been difficult because it causes the evolution of hydrogen gas by the reaction of contained metallic aluminum and alkaline water and results in the formation of porous and brittle solid. We found that the evolution of hydrogen gas is possible to be suppressed by improving the mixing method of the ingredients and controlling the amount of CaCl₂ in the mixture. Thorough mixing of the fly ash and cement powders with a drum-type mixer and subsequent addition of water at once scarcely induces the mechanical destruction of protective oxide layers surrounding metallic aluminum particles during the mixing process, so that the aluminum particles are protected from the attack of alkaline water. Water soluble CaCl₂ externally added and/or contained in the fly ash helps not only the homogeneous penetration of water into the powder mixtures but also reduces the alkalinity of water, so that it suppresses the evolution of hydrogen gas.