An inhibitory activity against ${alpha}-glucosidase$ was identified in extract of an edible lichen, Umbilicaria esculenta. The inhibitor was very stable retaining above 95% of its original activity when treated with heat, acid and alkaline conditions, and some hydrolytic enzymes. Partially purified inhibitor showed strong inhibition against disaccharide hydrolytic enzymes of mammalian and mold origin, but weak or no inhibition against polysaccharide hydrolytic enzymes except glucoamylase. The inhibitor suppressed elevation of blood glucose level in rats after oral administration of soluble starch or sucrose.