Toxic substance insolubilizer and method for toxic substance insolubilization

Provided is a toxic substance insolubilizer which has an average particle diameter of 2 mm or less and which is for insolubilization of a toxic substance, wherein the toxic substance insolubilizer comprises particles A containing calcium and magnesium, at least one of which forms an oxide, a carbonate, or a hydroxide, and particles B containing a phosphoric acid compound, and the insolubilizer has a molar ratio (P/Ca) of a calcium component in the particles A and a phosphorus component from the phosphoric acid compound in the particles B of 0.1 to 1.0.