A marker for determining the onset of periodontal disease and a marker for determining the progression stage of periodontal disease, each containing autoinducer-2. A screening method for identifying improving agents or preventive agents for periodontal disease using autoinducer-2 as screening target has been disclosed. 4-bromo-5-(4-methoxyphenyl)-2(5H)-Furanone, 3,4-dibromo-5-hydroxy-2(5H)-Furanone, 4-bromo-5-methoxy-5-(4-methoxyphenyl)-2(5H)-Furanone and 4-[[3-bromo-2,5-dihydro-2-(4-methoxyphenyl)-5-oxo-2-furanyl]oxy]-Benzoic acid have been described as medicaments and are useful for treating periodontal disease.
