The invention provides an enzyme having a lignan-hydroxylating activity, particularly an enzyme capable of catalyzing the reaction of transferring a hydroxyl group to a lignan, an enzyme capable of catalyzing the hydroxylation of piperitol to 9-hydroxylpiperitol or pinoresinol to 9-hydroxylpinoresinol. The invention provides a polypeptide having a lignan-hydroxylating activity; a polynucleotide encoding the same; a vector or transformant comprising the polynucleotide; a method for producing a polypeptide having a lignan-hydroxylating activity which comprises using the transformant; and so on. The transformant wherein the polynucleotide is expressibly introduced is useful for the hydroxylation of a lignan or for the production of a product using the same in the food sector and a variety of industry sectors.