A 66-year-old man was admitted to our hospital on suspicion of lung cancer with bone metastasis. He suffered multiple joint and muscle pain. 18F-Fluorodeoxy glucose positron emission tomography (FDG-PET) showed multiple accumulations in the lung, bones including the vertebrae, and mediastinal lymph nodes. Anti-human immunodeficiency virus (HIV) antibody was negative. Because Mycobacterium avium complex (MAC) was isolated from bronchial lavage fluid, bronchial wall, peripheral blood, and muscle abscess, he was diagnosed as having disseminated MAC infection. Although multidrug chemotherapy was initiated, his condition rapidly deteriorated at first. After surgical curettage of the musculoskeletal abscess, his condition gradually improved. As for etiology, we suspected that neutralizing factors against interferon-gamma (IFN-γ) might be present in his serum because a whole blood IFN-γ release assay detected low IFN-γ level even with mitogen stimulation. By further investigation, autoantibodies to IFN-γ were detected, suggesting the cause of severe MAC infection. We should consider the presence of autoantibodies to IFN-γ when a patient with disseminated NTM infection does not indicate the presence of HIV infection or other immunosuppressive condition.