A method for producing an optically active 3,5-dihydroxyhexanoic acid derivative by stereoselectively reducing an optically active 3-oxo-5-hydroxyhexanoic acid derivative is provided. The method, which requires neither an ultralow-temperature reactor, an incubator, nor protection of the 5-position hydroxy group, is simple and economical. An optically active 3,5-dihydroxyhexanoic acid derivative is produced by asymmetrical hydrogenation of an optically active 3-oxo-5-hydroxyhexanoic acid derivative catalyzed by an RuBr₂BINAP complex prepared from a ruthenium complex and a ruthenium-optically active phosphine complex, i.e., 2,2-bisdiarylphosphino-1,1-binaphthyl (BINAP), while using extremely inexpensive hydrogen as the reductant.