PURPOSE: To make fly ash harmless by making harmful heavy metals contained in fly ash generated in waste incineration insoluble by the addition of a chemical agent.

CONSTITUTION: Fly ash collected as dry powder, after being mixed with water to make the ash fluid, is mixed adequately with a liquid chemical agent in which a small amount of an organic polymer having reactive functional groups is mixed with liquid containing sulfide ions. The harmful heavy metals contained in the fly ash are contacted with the agent to form promptly a sulfidized heavy metal compound which is different from a polymeric chelate compound. By producing inorganic molecular crystals, fine particles produced are converted into big crystals which are an insoluble treated product.