During the pre-war days, a few cyanide gold mills had been gaining good results by substituting partly Aero Brand Cyanide made in America for NaCN. After the World War II, the domestic production of Ca (CN) 2 was commenced by carbide industries. The writers attempted to apply it to the cyanidation plant, and its foundamental research for six months proved to be satisfactory. Consequently, since August 1956, the over-all use of Ca (CN) 2 for the operation, resulted in our great profit by ecconomy of the reagent cost as well as the promotion in disolution of silver.The writers compared H2SO4 method with ZnSO4 method in the fundamental study to remove and recover various cyanide salts, which are unfavourable to the cyanidation of gold ore. It is found that the latter method has more advantage industrially.As further applications of Ca (CN) 2 to the non-ferrous metallurgy, by using it as dissolving reagent, the behaviour of various elements has been made clear in the experiments of metallic dross or flue dust, for which any appropriate metallurgical method has not been discovered at the smelters.