A 81-year-old woman was diagnosed as having diabetes mellitus (DM) at 58 years of age. She started insulin therapy the following year, but her blood sugar levels were poorly controlled. At the age of 75, she tested positive for the anti-GAD antibody (7.8 U/ml) and was diagnosed as having slowly progressive type 1 DM (SPIDDM), as well as vitiligo vulgaris. At 78 years of age, chronic thyroiditis was diagnosed after positive tests for anti-thyroid peroxidase antibody and anti-thyroglobulin antibody. At the age of 81, general fatigue and jaundice appeared concomitantly with severe anemia, with Hb levels at 5.2 g/dl. Low serum vitamin B12 levels and the finding of erythroblastic hyperplasia with megaloblasts in bone marrow led to the diagnosis of pernicious anemia. Anemia was alleviated by intramuscular injections of vitamin B12. The patient developed chronic thyroiditis, vitiligo vulgaris, and pernicious anemia concomitantly with SPIDDM, and was diagnosed as having polyglandular autoimmune syndrome type III. Attention should be paid to these potentially associated autoimmune diseases in daily practice during the follow-up of SPIDDM patients.