To obtain an R-(-)-mandelic monooxygenase gene that codes for a protein which has a specific amino acid sequence and shows R-(-)-mandelic monooxygenase activity, and is used for production of, mandelic acid which is useful as a material for pharmaceutical syntheses.

This R-(-)-mandelic monooxygenase gene, which codes for a protein that is composed of amino acids of the sequence shown by the formula, or amino acids of the sequence shown by the formula in which one or more amino acids are lost, replaced or added and, and has R-(-)-mandelic monooxygenase activity, and is useful for an enzyme able to efficiently produce expensive optically active mandelic acid useful as a material for pharmaceutical syntheses from its inexpensive racemic modification, is obtained by preparing a gene library by using genes of Pseudomonas putida A10L strain, introducing a plasmid of the gene library into a host cell, and selecting stocks that enrich the optically active mandelic acid.