A novel, optically active, quaternary ammonium salt with axial asymmetry which is useful as a phase-transfer catalyst with which a glycine derivative is stereoselectively alkylated to convert it into an optically active α-amino acid derivative. The quaternary ammonium salt is, for example, a compound represented by the formula (I). The quaternary ammonium salt is usable as a phase-transfer catalyst for stereoselectively alkylating a compound having a specific structure.