The patient was a man in his 70s. During the treatment for acute myeloid leukemia, abdominal pain and bloody stools appeared. A diagnosis of small intestinal ileus was made by computed tomography scan. Treatment with an ileus tube did not improve his condition, and enteroscopy revealed the presence of ileal ulcers. Based on histological examination, small intestinal mucormycosis was suspected, and thus, antifungal drugs were administered. However, the patient developed perforated peritonitis and underwent small intestine resection. He was finally diagnosed with small intestinal mucormycosis with the help of the resected specimen. The gastrointestinal form of mucormycosis rarely occurs, and small intestinal lesions are very rare. Enteroscopy was helpful in its diagnosis and treatment.