To provide a marker solving such problems that since there is substantially no subjective symptom for periodontitis, there are many cases that periodontitis is not examined by dentist until it is aggravated, maximally contributing to the elderly's teeth loss; however, there are substantially no techniques enabling periodontitis to be objectively diagnosed, with nothing but saying that the diagnosis of periodontitis is relied on conventional vidual determination; further, as for conventionally known periodontitis-causing bacteria, there are a variety of relationships with periodontitis, therefore periodontitis cannot be diagnosed properly based on merely observing specific relevant bacteria alone; and capable of objectively diagnosing periodontitis' progression, seriousness, risks, etc.

A method for diagnosing periodontitis involves using an AP24 periodontitis marker gene to objectively diagnose the disease.