This invention relates an .alpha.-glucosidase inhibitor mildly inhibiting .alpha.-glucosidase locally present in the micro-villus of small intestine, a composition principally comprising sugar and containing the inhibitor, a food, a sweetener and a feed. The inhibitor delays the digestion of starch, starch-derived oligosaccharides and sucrose, so that the inhibitor has an action of suppressing rapid increase in blood glucose level and an action of suppressing insulin secretion at a lower level. Thus, the inhibitor is useful for the prophylaxis of obesity and diabetes mellitus. The .alpha.-glucosidase inhibitor of the present invention is composed of nucleotide, nucleoside, or base as the structural component of nucleic acid and one or two or more digestible sugars selected from sucrose, starch and starch-derived oligosaccharide. The .alpha.-glucosidase inhibitor mildly inhibits the action of .alpha.-glucosidase as a digestive enzyme in small intestine, and has effects of suppressing rapid increase in blood glucose level and suppressing insulin secretion at a lower level. In combination with digestible sugars, the .alpha.-glucosidase inhibitor is applicable as foods, sweeteners and feeds.