Immunoglobulin G4 (IgG4)-related disease is an autoimmune disease that forms tumorous lesions. Several cases involving various organs are reported, however, IgG4-related disease involving appendix has not been reported yet. In this report, we presented a case of IgG4-related disease of appendix, which raised a suspicion of appendiceal tumor or usual appendicitis and, therefore, led to unnecessary surgical resection. IgG4-related disease should be considered in the differential diagnosis for a mass-like swelling of the appendix, in order to avoid unnecessary surgery.