Dipeptidyl peptide-4 (DPP-4) inhibitors prevent the degradation of glucagon- like peptide (GLP)-1 which increases the secretion of insulin and decreases the secretion of glucagon, thereby reducing blood sugar. DPP-4 inhibitor is clinically beneficial because it can safely be used by elderly individuals who are at increased risk of hypoglycemia. But We recently experienced a case of hypoglycemia with type 2 diabetes mellitus having taken metformin and vildagliptin. A 82-year-old man with type 2 diabetes having taken metformin 500 mg and vildagliptin 50 mg daily for 14 days was admitted with decreased mentality. At that time, blood glucose, insulin and C-peptid was each 26 mg/dL, 100 uU/mL and 25.52 ng/mL. His clinical course improved over 54 hours after infusion of dextrose solution. At improved state, insulin and C-peptide was each 11.2 uU/mL and 5.20 ng/mL. Insulin antibody was negative.