Disseminated nontuberculous mycobacterial infection is one of the opportunistic infections among patients with congenital and acquired immunodeficiency, including AIDS and some disease- or therapy-related conditions. Here we report a case of disseminated nontuberculous mycobacterial infection, which seems to be caused by an autoantibody against interferon-γ, a pivotal cytokine for combating Mycobacterium and other intracellular micro-organisms. The patient was a 74-year-old man who presented with high fever. His physical findings were unremarkable, but WBC count, CRP, and procalcitonin levels were elevated. Chest radiograph and CT revealed bilateral mediastinal lymph node enlargement without any consolidation. He was diagnosed as sepsis of unknown origin, but several courses of antibiotics were ineffective. Meanwhile, his sputum yielded Mycobacterium intracellulare, and a bone-marrow biopsy revealed epithelioid granuloma, leading to the diagnosis of disseminated nontuberculous mycobacterial infection. Afterward, his bone marrow yielded M. intracellulare. He had neither suffered from immunodeficient diseases nor taken any immunosuppressive medications, but anti-interferon-γ antibody was found in his serum, which appeared to be the culprit of his illness.