PURPOSE: To recover water-soluble proteins from waste water containing thereof, without using a coagulant, by electrolyzing the waste water with an insoluble electrode.

CONSTITUTION: Waste water containing water-soluble proteins discharged from the food processing factory, e.g. fish meal grinding mill, is electrolyzed using an electrode with does not disintegrate, decompose or dissolve during the electrolysis, e.g. a Pt electrode, a Pt-clad electrode, etc. The electrolysis is carried out (1) using separate electrolytic cells, and electrolyzing first in the cathode chamber and then in the anode chamber, or (2) using a single electrolytic cell and electrolyzing the waste water flowing from the cathode to the anode, or (3) using separate electrolytic cells and electrolyzing separately in the cathode chamber and the anode chamber, and then mixing the electrolyte in the cathode chamber and that in the anode chamber.