A strain of Streptomyces sp. (YS-221-B) extracellularly produced an inhibitory substance for $alpha$-D-Glucosidase. The substance was purified 96-fold from culture filtrate by dialysis, heat treatment, adsorption on active carbon, Bio-Gel P-10 and Sephadex G-75 column chromatography with yield of 9.2%. The substance was stable in pH range from 7.0 to 11.0 at 37$^{circ}C$, and a treatment at 10$0^{circ}C$ for 20 min diminished only 15% of the original activity. The inhibitor was not inactivated by the treatment of $alpha$-, $eta$-amylases, glucoamylases, trypsin and chymotrypsin but inactivated by pyoteases from Streptomyces griseus and Tritirachium album.