Insulinoma is the most common type of islet cell tumor of the pancreas and its incidence is estimated at four per one million persons per year. Although fasting hypoglycemia is the typical presentation, cases with postprandial hypoglycemia have rarely been reported. A 48-year-old woman was admitted to our department for evaluation of hypoglycemia. Laboratory data suggested a state of postprandial endogenous hyperinsulinemic hypoglycemia. Abdominal computed tomography revealed a mass lesion measuring 20 mm at the distal pancreas. A decision was made to perform an arterial calcium-stimulated venous sampling for excluding nesidioblastosis coexisting with a pancreatic incidental mass. After that the patient was referred to surgery. Pathological examination revealed a low-grade well-differentiated neuroendocrine tumor with regional lymph node metastasis. Herein, we report a case of malignant insulinoma presenting with postprandial hypoglycemia and discuss the differential diagnosis and possible mechanisms of postprandial hypoglycemia in insulinoma cases.