PURPOSE:To prepare the titled compound useful as an intermediate for an antimicrobial agent in simple steps, by reacting a readily available 2-perfluoroalkylpropanal with an N-acylglycine as raw materials in the presence of a base and acetic anhydride. CONSTITUTION:A 2-perfluoroalkylpropanal of formula I (Rf is perfluoroalkyl) is reacted with an N-acylglycine of formula II (R is substituted or unsubstituted alkyl or aromatic group) in the presence of a base, preferably lead acetate, and acetic anhydride, preferably in an ethereal solvent, e.g. tetrahydrofuran, at 50-150 deg.C to give an azlactone derivative of formula III, e.g. azlactone of alpha-benzoylamino- beta-(1-trifluoromethyl)ethylacrylic acid. The compound of formula I is obtained from a 1-perfluoroalkylethylene by the oxo reaction.