The purpose of the present invention is to provide a water-soluble polysaccharide which is less expensive and enables the production of an acidic protein food or drink, said acidic protein food or drink being stable in an acidic pH range at the isoelectric point of protein or higher, even by using a starting material such as a root vegetable. A water-soluble polysaccharide, which is extracted from a starting material derived from a root vegetable in a wet state with hot water under acidic conditions of pH 2.7-3.7, can well stabilize an acidic protein food or drink in a pH range at the isoelectric point of protein or higher.