PROBLEM TO BE SOLVED: To provide an excellent catechol-O-methyltransferase inhibitor having high safety.SOLUTION: The catechol-O-methyltransferase inhibitor comprises, as an active ingredient, one or more selected from: lactoferrin a heat-treated product of lactoferrin an acid treated product of lactoferrin an alkali treated product of lactoferrin a hydrolyzate of lactoferrin a treated product of lactoferrin treated by two or more treatments in combination selected from a heat treatment, an acid treatment, an alkali treatment and a hydrolysis treatment and a peptide composed of an amino acid sequence set forth in SEQ ID No:1 or 3 and/or a peptide or protein containing the peptide.
