[Problem] The purpose of the present invention is to provide a natural polymeric emulsifier improved in so-called “emulsifying power”, said natural polymeric emulsifier being capable of forming, even by the addition thereof in a small amount, an oil-in-water emulsion which has a small emulsified particle diameter and a uniform particle size distribution and which rarely suffers from the aggregation or coalescence of emulsified particles even when has undergone heating, long-term storage, aging or the like.[Solution] The purpose can be achieved by using water-soluble soybean polysaccharides which have a methyl ester content of 30% or below relative to the uronic acids and an acetate content of 1 wt % or higher in terms of free acetic acid and relative to the water-soluble soybean polysaccharides. The water-soluble soybean polysaccharides can be manufactured by subjecting raw material water-soluble soybean polysaccharides to methyl ester removal treatment and thereafter acetate-forming esterification treatment.
