An industrially applicable process for producing cis-3-hydroxy-L-proline, which is useful as a raw material for medicines or as an additive to foods. In the process, L-proline is converted into cis-3-hydroxy-L-proline in the presence of an enzyme source which is derived from a microorganism belonging to the genus Streptomyces or Bacillus and which catalyzes hydroxylation of L-proline into cis-3-hydroxy-L-proline, a divalent iron ion and 2-ketoglutaric acid, in an aqueous medium, and the produced cis-3-hydroxy-L-proline is collected from the aqueous medium. A novel enzyme L-proline-3-hydroxylase, a gene of L-proline-3-hydroxylase which is useful for the process, a transformant containing the gene, and a process for producing L-proline-3-hydroxylase using the transformant.