The present invention provides an inhibitor of an autoimmune response to a periodontal bacterial enzymatic degradation product of keratin in gingival epithelium in a mammal having a periodontal bacterium in the oral cavity, containing a substance having affinity to the keratin or a degradation product thereof and/or a substance having affinity to an autoantibody to the degradation product, an agent for the prophylaxis and/or treatment of a periodontal disease and/or a complication thereof； a RANKL expression inhibitor containing a substance having affinity to the keratin or a degradation product thereof； and a method of diagnosing a periodontal disease including detecting the keratin or a degradation product thereof and/or an autoantibody thereto.
