The inventor discovered that .gamma.-GTP (.gamma.-glutamyl transpeptidase) was scarcely contained in the gingival crevicular fluid of patients with gingivitis, but was contained in the gingival crevicular fluid of patients with periodontitis. Thus, the inventor provides a determination method of development of periodontitis by detecting or measuring .gamma.-GTP contained in the gingival crevicular fluid. Also, this method can be applied to the determination of the development of peri-implant inflammation.