Porphyromonas gingivalis is an anaerobic Gram-negative bacterium regarded to be an etiological agent of periodontitis. Outer membrane vesicles (OMVs) of P. gingivalis carry a wide range of virulence factors such as lipopolysaccharide (LPS) and gingipains. Recently, P. gingivalis is regarded as keystone pathogen for development of periodontal disease. Therefore, there is a wide interest in developing a vaccine against P. gingivalis and P. gingivalis is not bad as a first choice of periodontopathic bacteria in the primary process of vaccine development. This book summarizes the recent results obtained in I and my colleagues concerning the possible immunological functions of OMVs produced by P. gingivalis. The vaccine animal model using P. gingivalis OMVs successfully elicited the immune responses that produced abundant antibodies in serum against a wide range of virulence factors including LPS, gingipains, and fimbriae. These findings suggest the possible applicability of the system for development of a periodontal vaccine.