PURPOSE: To prevent the occurrence of dregs by heating, by bringing raw soy sauce into contact with an immobilized protease prepared by adsorbing it directly to a porous adsorbent, so that thermally denatured protein is treated with it.

CONSTITUTION: A protease (e.g., pepsin, trypsin, alkali protease of yeast fungus, etc.) is brought into contact with a porous adsorbent (e.g., styrenic synthetic adsorbent, synthetic adsorbent of methacrylic acid ester, etc.) having hydrophobic bond strength, and adsorbed on it, to give an immobilized protease, which is brought into contact with raw soy sauce under heating or immediately after heating. Thermally denatured protein is treated with the immobilized protease, so that occurrence of dregs by heating is prevented.