The present invention addresses the problem of providing a novel substance having a natural immunostimulatory action. Provided are: a polysaccharide having a natural immunostimulatory action, characterized by comprising, as constituent saccharides, 25-50 parts by mol of galacturonic acid, 15-50 parts by mol of galactose, 0-7 parts by mol of glucose, 0-30 parts by mol of arabinose, 0-6 parts by mol of xylose and 3-15 parts by mol of rhamnose, and having, as the main chain, a polygalacturonic acid chain consisting of galacturonic acid residues linked by a-1,4-bonds； a natural immunostimulant which comprises the polysaccharide as an active ingredient； and a food or a drink comprising the polysaccharide.
