To completely stabilize a heavy metal-containing waste by incorporating a latent hydraulic material and a boron compound to prepare a treating agent for the heavy metal-containing waste.

The latent hydraulic material used as an essential component does not cause hydration reaction with water alone and, for example, causes hydration reaction with the addition of a stimulating agent and for example, a blast furnace granulated slag or a converter slag is used. In the case of using the blast furnce granulated slag as the latent hydraulic material, one having a particle size corresponding to ≥4,000 cm2/g Blaine specific surface area is used. The boron compound to be used is preferably a borate compound, particularly sodium tetraborate is preferable in the view point of heavy metal stabilizing effect and profitability. The boron compound is converted to a hardly water soluble heavy metal boride or heavy metal borate by allowing boron ion or boric acid ion generated in the presence of water to react with the metal ion in the heavy metal-containing waste to stabilize the heavy metal.