A 38-year-old man was admitted because of exacerbation of erythema. Full-body computed tomography (CT) scanning revealed a tumor mass in the tail of the pancreas; CT and magnetic resonance imaging (MRI) scans confirmed the presence of a spherical mass. In contrast CT scans, although the contrast was gradually increased, no strong contrast differences were observed between the tumor and the surrounding tissue. Endoscopic ultrasonography (EUS) revealed a spherical low-density mass. Blood test results revealed that the patient had a high glucagon level. We diagnosed glucagonoma syndrome on the basis of the above results and resected the tail of the pancreas. Pathological analysis revealed that the tumor cells had proliferated in ribbon-like, cord-like structures. Immunostaining results were positive for glucagon, which confirmed our diagnosis.