A process for producing β-lactam derivatives, characterized by treating a β-lactam derivative having a protected carboxyl group and represented by the general formula (1): A-COO-X (wherein A is a β-lactam derivative residue; and X is a benzyl group which may have an electron-donating group as a substituent on the phenyl ring, or a diphenylmethyl group which may have an electron-donating group on the phenyl ring) with an aluminum halide in an aliphatic ether solvent to obtain a β-lactam derivative of the general formula (2): A-COOH (wherein A is as defined above), whereby the leaving group X can be recovered as X-OH and reused.