PURPOSE: To produce the titled substance useful as an intermediate for the production of phenylalanine, under binary phase condition, in high yield, without using an organic acid as a solvent, by using a halogen-containing ethylbenzene derivative as a raw material, and thermally hydrolyzing the derivative in an aqueous solution of a mineral acid under stirring.

CONSTITUTION: The halogen-containing ethylbenzene derivative of formula I (W is cyano, amidocarbonyl, or lower alkoxycarbonyl; X is halogen), e.g. α-halogeno-β-phenylpropionitrile, is hydrolyzed by heating under vigorous agitation in an aqueous solution of a mineral acid at about 80W200°C. The halogen-containing ethylbenzene is preferably emulsified with the aqueous solution of mineral acid by the vigorous agitation during the reaction. The mineral acid is e.g. hydrochloric acid, sulfuric acid, etc., and its amount is preferably ≥3mol per 1mol of the compound of formula I used as a raw material. The concentration of the mineral acid in the aqueous solution of mineral acid is especially preferably ≥20wt%.