The present invention addresses the problem of providing an excellent pharmaceutical for preventing gingivitis or treating gingivitis. The present invention provides a pharmaceutical for preventing or treating gingivitis, the pharmaceutical having as an active ingredient a hydantoin derivative represented by general formula (I) (in the formula, X signifies a halogen atom or hydrogen atom, and R1 and R2 either simultaneously or separately represent a hydrogen atom or optionally substituted C1-6 alkyl group) or a pharmaceutically acceptable salt thereof.本發明所欲解決之課題在於提供一種牙齦炎之預防或牙齦炎之治療優異的醫藥。本發明係提供一種牙齦炎之預防或治療用醫藥，其以通式(I)(式中，X表示鹵素原子或氫原子，R1及R2同時或分別表示氫原子或亦可經取代之C1-6烷基)所表示之乙內醯脲衍生物或其藥理學所容許之鹽為有效成分。
