Insulinoma is a rare functional islet cell tumor of the pancreas which is characterized by fasting hypaglycemia. The symptoms are incidious in onset and show dramatic relief by oral or intravenous administration ot glucose. A 54-year-old woman presented with intermittent loss of conciousness. In prolonged fasting test, she was severely hy poglycemic. The serum insulin and C-peptide levels were elevated inappropriately. At surgery a ovoid shaped 1,3 x 1.0 cm sized mass at tail of pancreas was palpated. Distal pancreatectomy was done. Pathologic examination of the resected pancreas by light microscope, immunohistochemical stain and electron microseope sbowed the islet cell tumor, glandular pattern, of pancreas. Lnsulin-secreating granules with Zn-Insulin crystals surrounded by halo were confirmed by electron micro- scope. A brief review of the literature was made.