The invention relates to a GLP-1 secretagogue and an inhibitor of postprandial rise in blood glucose, containing .kappa.-casein as an active ingredient, and a food or drink for promoting GLP-1 secretion and an inhibitory food or drink of postprandial rise in blood glucose, containing a milk-derived casein protein wherein .kappa.-casein accounts for 60% by mass or more of the milk-derived casein.