Disclosed is a novel water-soluble soybean polysaccharide which is produced by imparting a function of stabilizing a protein in a pH range around the isoelectric point of the protein to a water-soluble soybean polysaccharide. Also disclosed is a novel dispersion stabilizer which enables to stabilize the dispersion of a protein in a pH range around the isoelectric point of the protein. Further, disclosed is a acidic protein food/beverage comprising the dispersion stabilizer. A crosslinked product of a water-soluble soybean polysaccharide extracted from a soybean seed raw material can improve the dispersion stability of a milk protein in a pH range around the isoelectric point of the protein. By using the water-soluble soybean polysaccharide, an acidic protein food/beverage having a low viscosity can be produced.