Abstract: The present invention provides an immobilizing agent for making harmless the detrimental metals contained in fly ash formed from an urban dust incinerating plant and a method for immobilizing the fly ash. The treating agent according to the present invention contains .alpha. starch and/or dextrin (component A), sodium silicate and/or a hydrogen phosphate compound (component B) and an allophane having a molecular ratio of SiO.sub.2 /Al.sub.2 O.sub.3 of not lower than 2 (component C). It may also further contain cement, bentonite, calcined gypsum and fly ash formed from a coal thermoelectric power plant. The mixing ratio of the components A, B and C is preferably 100 parts by weight: 20 to 160 parts by weight: 40 to 180 parts by weight respectively. The treating agent is mixed with the above-mentioned fly ash and water is added to it and the mixture is milled and then used.