A collagen peptide mixture containing three or more kinds selected from Glu-Hyp-Gly, Glu-Hyp, Leu-Hyp-Gly, Pro-Ala, Ser-Hyp, Ala-Hyp-Gly, chemically-modified substances thereof and pharmaceutically acceptable salts thereof, and at least one peptide selected from the group consisting of Glu-Hyp-Gly, Glu-Hyp, Leu-Hyp-Gly, Pro-Ala, Ser-Hyp, Ala-Hyp-Gly, Pro-Hyp-Gly, Leu-Hyp, Ile-Hyp, Ser-Hyp-Gly, Gly-Pro-Hyp, (Pro-Hyp-Gly)5, Pro-Hyp, Hyp-Gly, Pro-Gly, Pro-Pro and Ala-Hyp or a chemically-modified substance thereof or a pharmaceutically acceptable salt thereof have DPPTV inhibitory activity and/or GLP-1 secretion accelerating activity, and hence are effective as a therapeutic or preventive agent or the like for diabetes.
