To produce an α-anilinonitrone derivative effective in reducing terminal carboxyl groups in a polyester, excellent in hydrolytic resistance and useful for papermaking canvases, tire cords, fabrics, etc., for medical use.

This anilinonitrone derivative is represented by formula I (R2, R12, R3, R13, R4 and R5 are each a 1-7C alkyl; R6, R16, R7, R8, R18, R9, R19, R10, and R11 are each a 1-16C alkyl or H), e.g. α-3,5-di-t-butylanilino-N-3,5-di-t- dutylphenylnitrone. The derivative represented by formula I is obtained as a by-product of carbodiimide by reacting an alkyl-substituted phenyl isocyanate in an aromatic hydrocarbon solvent in the presence of a phosphorus catalyst for synthesizing the carbodiimide. A polyester with a low content of terminal carboxyl groups is prepared by bringing at least one α-anilinonitrone derivative selected from compounds represented by formulas I and II into contact with the polyester containing the terminal carboxyl group.