By simultaneously carrying out an isomerization reaction at position 2 of monatin in different configurations at positions 2 and 4 in the presence of an aldehyde under a condition of pH 4 to 11 in a mixture solvent of water and an organic solvent, and the crystallization of monatin in the same configurations at positions 2 and 4 or a salt thereof, monatin useful as a sweetener, particularly optically active monatin can efficiently be produced.