A polysaccharide comprising galactose, glucose, rhamnose, and pyruvic acid as constituents, wherein the galactose, glucose, and rhamnose are contained in a molar ratio of 4 : 2 : 1, and the pyruvic acid is contained in an amount of 4 to 7 wt%. The polysaccharide can be obtained by culturing Bifidobacterium longum JBL05 (NITE BP-82) under anaerobic conditions.