The present invention provides a vaccine for the prophylaxis or treatment of diabetes using a polypeptide containing the amino acid sequence shown by SEQ ID NO: 2 or an amino acid sequence of a non-human mammal, which corresponds to SEQ ID NO: 2, and the like as an immunogen, which induces a neutralizing antibody to DPP-4, and a prophylactic or therapeutic agent for diabetes, which contains a DDP-4 neutralizing antibody that recognizes a partial amino acid sequence of the aforementioned DDP-4.