We report a case of reactive hypoglycemic coma in a 77-year-old man. Seven months after partial gastrectomy for early gastric cancer, he presented with syncopal attacks and seizure. His plasma glucose and insulin levels at coma were 18 mg/dl and 27μU/ml. Insulinoma was neglected with computerized tomography, magnetic resonance imaging and angiography. Because dietary control was insufficient, oral diazoxide therapy was done. But diazoxide did not protectthe overresponse of the insulin and reactive hypoglycemia in 75 g GTT. Octreotide (100 micrograms IM) inhibited insulin release and prevented hypoglycemia. Acarbose delayed the response of insulin butdid not inhibited insulin release. However, acarbose also prevented reactive hypoglycemia. We concluded that acarbose is an effective therapy for reactive hypoglycemic coma.