A novel D-serine quantification method that can overcome various disadvantages of a conventional D-serine quantification method; a novel enzyme that can be used in the D-serine quantification method; a gene encoding the enzyme; and the like. Specifically, a novel D-serine dehydratase including (a) a protein having an amino acid sequence set forth in SEQ ID NO: 1 or (b) a protein having an amino acid sequence homologous to the amino acid sequence set forth in SEQ ID NO: 1 and having a D-serine dehydratase activity; and a D-serine quantification method including the steps of reacting a sample with the enzyme, quantifying ammonia or pyruvic acid produced by the reaction, and calculating the amount of D-serine in the sample based on a value produced by the quantification.