A 45-year-old man with slowly progressive insulin-dependent diabetes mellitus and diabetic nephropathy was admitted to our hospital complaining of intense back pain. Computed tomography (CT) showed multiple lung nodules in both lung fields, swelling of hilar and mediastinal lymph nodes, and hepatic enlargement. Magnetic resonance imaging of the spine showed multiple osteolytic lesions without abscess. Broad-spectrum antibiotics were administered on suspicion of disseminated infection, but this proved ineffective and his general condition deteriorated. Disseminated Mycobacterium avium complex (MAC) was diagnosed after detection of M. avium from bronchoscopic and bone-marrow biopsies. His condition and the pathological findings in multiple organs improved with antibiotics for MAC. Interferon-γ neutralizing autoantibody was detected from a blood sample and was considered a critical risk factor for his disseminated MAC disease.