The objectives of the present invention are: to provide a dispersion stabilizer the suppresses precipitation/clumping of protein under acidic conditions, particularly the entirety of a wide pH range of 3.2-4.8, particularly 3.4-4.5, which is more acidic than the vicinity of the isoelectric point of the protein； and to provide an acidic protein beverage that has low viscosity and a refreshing taste. By means of using a pectic polysaccharide that, among the pectic saccharides included among the cell wall saccharides of legumes, has a degree of methyl esterification of no greater than 45% and preferably no greater than 30%, and a diameter of molecules having a star structure of over 100 nm and no greater than 200 nm, it was found to be possible to stabilize a dispersion of protein particles in a wide pH range of 3.2-4.8, particularly 3.4-4.5, and the present invention was achieved. Of the constituent components of the polysaccharide, the macromolecule component having a molecular weight of at least 10,000 preferably has an average absolute molecular weight of 50,000 to 100,000 and a molecular rotation radius in an aqueous monodispersed state of 25-40 nm.
