PROBLEM TO BE SOLVED: To provide a collagen decomposition inhibitor comprising a herbal medicine as an active ingredient, and to provide a periodontitis therapeutic drug containing the collagen decomposition inhibitor as an active ingredient.SOLUTION: A herbal medicine having properties of (1) suppressing shrinkage of collagen gel and/or (2) leaving a large amount of collagen remaining in collagen gel is found by a screening method for collagen decomposition inhibitors. A collagen decomposition inhibitor is obtained by using the herbal medicine thus found as an active ingredient. A periodontitis therapeutic drug is obtained by using the collagen decomposition inhibitor as an active ingredient.
