The present invention relates to a composition for preventing or treating immune diseases through the suppression of B cell activity induced by metformin. More particularly, the present invention relates to a composition comprising a metformin compound or a pharmaceutically acceptable salt thereof as an active ingredient for preventing or treating immune diseases, wherein the composition is characterized by suppression or reduction of B cell activity which is a cause of disease. The present invention can be valuable in the use thereof for various autoimmune diseases as an immunosuppressant which can prevent or treat immune diseases by suppressing or reducing B cell activity and Th17 cell activity, which are causes of disease, or by promoting or increasing regulatory T cell activity.