To obtain a heavy metal insolubilizer capable of insolubilizing heavy metals in a soil, rarely giving any damage to the soil nor resulting in a new contamination problem.

Since the insolubilizer of heavy metal in the soil according to the present invention is characterized in that a humic material is dissolved in water, heavy metals in the soil can be insolubilized with a simple method that the soil contaminated with heavy metals is impregnated with this heavy metal insolubilizer. Moreover, as the fumic material is a natural compound that can be obtained by soaking leaf mold, dead leaf, peat moss, and the like, in water for a predetermined time, the soil is not damaged by the fumic material even if the soil is impregnated with the material.