[Problem] To provide a method for producing useful materials, wherein said method produces a nontoxic useful material, e.g., a fertilizer or soil conditioner, by subjecting a heavy metal-containing organic waste to carbonization of the organic material while detoxifying the heavy metal. [Solution] The present invention is a method for producing useful materials in which a carbonized material-containing useful material is produced by treating an organic waste in which heavy metal is admixed, wherein the method characteristically comprises: a tobermorite starting material addition step of adding, to an organic waste in which heavy metal is admixed, a Ca component starting material and an SiO2 component starting material, which are tobermorite starting materials, in amounts sufficient to sequester the admixed heavy metal in the tobermorite crystal; a carbonized material formation step of performing a dry carbonization treatment on the organic waste to which these tobermorite starting materials have been added, to carbonize the organic waste and form a carbonized material; a heavy metal sequestration step of introducing, into a reactor and while maintaining the high-temperature condition in the previous step intact, the aforementioned tobermorite starting materials and the carbonized material formed in the carbonized material formation step, and slowly cooling while stirring in the presence of water, to form a tobermorite crystal mineral that sequesters the heavy metal admixed in the organic waste and form a slurry in which the carbonized material and tobermorite crystal mineral are dispersed; and a useful material production step of drying or dewatering the slurry formed in the previous step to produce a useful material containing carbonized material in which the tobermorite crystal mineral is admixed.