The purpose of the present invention is to provide an excellent dipeptidyl peptidase-IV inhibitor and others. A dipeptidyl peptidase-IV inhibitor, a hypoglycemic agent or an agent for preventing vascular endothelial dysfunction, each of which contains a peptide comprising Met-Lys-Pro as an active ingredient； and a method for preventing, ameliorating or treating a disease associated with dipeptidyl peptidase-IV, a disease associated with a hyperglycemic state, diabetes or a disease associated with vascular endothelial dysfunction or vascular dysfunction using a peptide comprising Met-Lys-Pro as an active ingredient. Preferably, the disease associated with dipeptidyl peptidase-IV is selected from hyperglycemia, diabetes, a diabetic complication, vascular endothelial dysfunction and vascular dysfunction.
