The present inventors focused on the lotus plant and prepared lotus extracts. When .alpha.-glucosidase and its substrate sucrose were reacted with the lotus extracts to examine the effect of adding the extracts on the enzymatic reaction, the lotus extracts were found to have .alpha.-glucosidase inhibitory activity. Furthermore, when the lotus extract and sucrose were administered to mice and their blood glucose levels were measured, the lotus extracts were found to suppress blood glucose levels. More specifically, the present inventors discovered that the lotus extracts have .alpha.-glucosidase inhibitory activity and can be used to control blood glucose levels.