By adding a basic substance, for example, a hydroxide or an alcoholate of alkali metals such as lithium hydroxide, sodium hydroxide, potassium hydroxide, sodium methylate and sodium ethylate at a rate of 0.05 to 40% by weight, preferably 0.5 to 20% by weight, based on the starting trinitrile, at the same time, there may be obtained more desirable results in that the amount of catalyst necessary to