PURPOSE: To obtain an industrial antifoaming agent which is fastacting and persistent irrespective of the kind of the generated bubbles by incorporating the reactional product obtained by adding an alkylene oxide to the mixture of natural oil, water and a catalyst to bring about a reaction into the agent.

CONSTITUTION: An alkylene oxide such as ethylene oxide and propylene oxide is added by 5-100 mols to one mol of the mixture of natural oil, e.g. coconut oil and palm oil, water and a hydroxide of alkali material, alkalimetal as catalyst to bring about a reaction. Consequently, transesterification and an alkylene oxide addition reaction take place simultaneously, and a mixture consisting of a special structure contg. a fatty acid residue at the terminal of the molecule is obtained. Bubbles are eliminated by the antifoaming agent thus obtained irrespective of the kind and temp. of the bubbles, hence productivity is improved, and pollution is prevented.