The subject of the present invention is to provide a therapeutic drug and a treatment method for periodontal disease pathogens that have not been effective by the previous photodynamic therapy (PDT). Regarding the treatment method of the present invention, the following formula (I) will be included (In the formula, R 1 represents a hydrogen atom or an acyl group, and R 2 represents a hydrogen atom, an alkyl group, a cycloalkyl group, an aryl group or an aralkyl group) The compound represented or a pharmaceutically acceptable salt thereof for the treatment of periodontal disease The medicine is administered to the periodontal diseased area, and then irradiated with light with a wavelength of 360 nm to 700 nm.本發明之課題在於提供一種對於藉由先前之光動力療法(PDT)未有效果之牙周病病原菌亦可實施PDT之治療藥及治療方法。關於本發明之治療方法，將包含下述式(I)(式中，R1表示氫原子或醯基，R2表示氫原子、烷基、環烷基、芳基或芳烷基)所表示之化合物或其藥學上容許之鹽之牙周病之治療藥投予至牙周病患部，其後照射360 nm～700 nm之波長之光。
