PURPOSE: To execute the subject synthesis of cyanoacetic acid and ketone in one stage in an industrially advantageous manner by uniformly adding a specific amt. of acetic acid to ketone as a soln. together with cyanoacetic acid at specific temp. so as to generate a regulated waste gas flow at this time.

CONSTITUTION: In Knoevenagal synthesis reacting cyanoacetic acid with ketone, for example, cyclohexanone or cyclododecanone by using a general catalyst, 1-10 mol, pref., 1-5 mol, especially, 1.1 mol of acetic acid is used per 1 mol of ketone to prepare a soln. together with cyanoacetic acid, and this soln. is uniformly added to ketone at 100-160°C, pref., at about 130-140°C in the case of cyclododecane or at about 100-120°C in the case of cyclohexane so as to generate a regulated waste gas flow at this time and this reaction is industrially executed in one stage without generating a problem of disposal treatment and the danger from an aspect of safety.