The present invention provides a plant-derived carbohydrase inhibitor, wherein the inhibitor is effective for preventing or alleviating diabetes, or preventing obesity, and foods, drinks, and medicines containing the same. The present invention is accomplished by use of an .alpha.-amylase inhibitor or an .alpha.-glucosidase inhibitor as a carbohydrase inhibitor that is extracted from an entire or a part of fagaceous plants using water, another polar solvent, or a mixture thereof. The present invention can also be accomplished by adding the carbohydrase inhibitor to a food or medical composition as an active ingredient for delaying saccharide digestion or absorption, suppressing a rise in postprandial blood glucose levels, or preventing obesity.