It is intended to provide a soybean protein material which is excellent in solubility, stability, emulsifying properties and gel-forming properties under acidic conditions and thus advantageously usable in acidic foods, its production process, and acidic foods using the soybean protein material. A solution containing soybean protein is subjected to a treatment for eliminating or inactivating polyanionic substances contained therein and/or adding a polycationic substance and then heated at above 100.degree. C. under acidic conditions. Thus, a soybean protein having a high solubility under acidic conditions and thus being appropriately usable in acidic foods can be obtained. By using this protein, protein foods in an acidic region can be provided. Further, by combining the above-described treatment with a protease-digestion treatment, a soybean protein hydrolysate having a high solubility in an acidic region can be efficiently obtained.