The present invention provides an enzyme having an activity of transferring a methyl group to lignans (a lignan methylation activity) (e.g., a protein comprising the amino acid sequence of SEQ ID NO:2 or variants thereof) a gene encoding the enzyme (e.g., a polynucleotide comprising the nucleotide sequence of SEQ ID NO:1 or variants thereof) a method for producing methylated lignans using the gene and so on.