A 51 year-old man was admitted to the hospital with fever and general fatigue. He was diagnosed as acute lymphoblastic leukemia after examination showed the existence of lymphoblasts in the bone marrow. Remission induction therapy caused severe suppression of the bone marrow. He suddenly complained of left chest pain, and a chest X-ray showed an abnormal shadow in the left upper lobe. Pulmonary fungal infection was suspected and intravenous amphotericin B was administered. The abnormal shadow on the chest X-ray improved but remained. Lobectomy of the left upper lobe was then performed. Histology of the resected lobe showed non-septate hyphae, and mucormycosis was diagnosed. Anti-leukemic chemotherapy was again bigun, but no relapse of mucormycosis was seen.