Objects of the present invention are to provide a dispersion stabilizer for suppressing aggregate precipitation of protein under acidic conditions, particularly in the entire wide pH range of pH 3.2 to 4.8, particularly of pH 3.4 to pH 4.5, which is more acidic than the vicinity of the isoelectric point of the protein, and to provide an acidic protein beverage of low viscosity and with a refreshing taste. Among the pectic saccharides included in the cell wall saccharides of beans, by use of a pectic polysaccharide whose degree of methyl esterification is 45% or less, preferably 30% or less, and the diameter of molecules having a star structure is from more than 100 nm and equal to or less than 200nm, it has been found to be possible to stabilize dispersion of protein particles in a wide pH range of pH 3.2 to 4.8, particularly pH 3.4 to pH 4.5, and the present invention has been completed. Among the constituent components of the polysaccharides, it is preferred that a polymeric component whose molecular weight is 10,000 or more has a molecular rotation radius of 25 nm to 40 nm when dispersed in an aqueous medium and an average absolute molecular weight of 500, 000 to 1,000,000.
