A 73-year-old man was admitted with high fever and right chest pain. Chest X-ray showed a rapidly growing mass shadow in the right lower lung field. The patient had been in remission for malignant lymphoma and had developed interstitial pneumonia and diabetes mellitus following 1 year of corticosteroid therapy. His illness was diagnosed as invasive aspergillosis because of a high level of beta-D-glucan and cultured Aspergillus fumigatus in the sputum. He was treated with a combination of micafungin and itraconazole. However, because these agents did not improve his clinical condition, transbronchial lung biopsy was performed. Histologically, Mucor hyphae were detected in these specimens. Micafungin and itraconazole were stopped and infusion of liposomal amphotericin B was initiated. Because his condition worsened, a right lower lobectomy was performed. Rhizopus Oryzae was detected in the lung tissue. We report a case of pulmonary mucormycosis in which mixed infection with A. fumigatus was suspected. Pulmonary mucormycosis is a life-threatening infection in which it is rare that an antemortem diagnosis is established and organisms are isolated. We believe diagnostic tests should be performed aggressively, even when pulmonary aspergillosis is suspected.