Extract of water-soluble polysaccharide (CFWx), showing inhibiting activity on ${alpha}-glucosidase$, was prepared from the fruiting bodies of Cordyceps militaris by hot-water extraction, and ethanol precipitation. Chemical characteristics of CFWx were as follows: carbohydrate content 30% including 16% of uronic acid; 51% protein content; monosaccharide composition, Man:Glu:Gal (30:43:27); molecular weight $3-5{ imes}10^4$. CFWx was further purified by ion-exchange, gel-permeation, and affinity chromatography and $CFWx-AH-{alpha}$ fraction was isolated. Fundamental structure of $CFWx-AH-{alpha}$ was deduced as ${alpha}-(1{ o}4$)-D-glucan with ${alpha}-(1{ o}3$)- and/or ${alpha}-(1{ o}6$)-D-glycosidic side chains based on methylation analysis.