The present invention aims to provide a DPP-4 inhibitor that is obtained by using a food as a raw material and that is suitable for oral ingestion from the viewpoints of flavor and absorbability, and a composition for the prevention and/or amelioration of diabetes which contains the DPP-4 inhibitor. The present invention provides a DPP-4 inhibitor obtained by treating an azuki bean or a kidney bean with a microorganism or a proteolytic enzyme produced by the microorganism. In particular, a preferable DPP-4 inhibitor can be obtained by hydrolyzing an azuki bean with a koji mold or a proteolytic enzyme derived from the koji mold to fragment a protein in the azuki bean.