On the other hand, when hydrogen cyanide is used as a cyanide compound and hydrazine hydrate is employed as hydrazines, a buffer solution of an inorganic salt, such as sodium bicarbonate, sodium carbonate, sodium formate, sodium acetete, sodium hydrogen phosphate and sodium dihydrogen phosphate, had better be added to the reaction medium beforehand in an amount of from 0.1 to 2.0 percent by weight