The present invention has an object to provide a mouthwash composition for preventing and/or treating periodontal diseases safely and effectively. The present invention solves the above object by providing a mouthwash composition for preventing and/or treating periodontal diseases, characterized in that it comprises, as an effective ingredient(s), one or more members selected from the group consisting of adenosine N1-oxide, 3′-glucosyladenosine N1-oxide, 5′-glucosyladenosine N1-oxide, 5′-adenosine diphosphate N1-oxide, and 5′-adenosine triphosphate N1-oxide but does not contain any saccharide assimilable by periodontal disease bacteria.