An α-glucosidase inhibitor, which is naturally present in the microvillus of the small intestine, delays the digestion of starch, starch-derived oligosaccharides and sucrose. This α-glucosidase inhibitor can be incorporated into foods, and is useful for the prophylaxis of obesity and diabetes mellitus. It mildly inhibits the action of α-glucosidase as a digestive enzyme in the small intestine, suppresses the rapid increase in blood glucose level and suppresses insulin secretion at a lower level. The inhibitor consists of a nucleotide, nucleoside or base as the structural component of nucleic acid and more than one digestible carbohydrate selected from sucrose, starch and starch-derived oligosaccharide.