The pH value of excess sludge taken out from a biological oxidation tank (2) is adjusted to 8 to 14, and the pH-adjusted sludge is injected into a solubilizing device (3). The sludge taken out from the solubilizing device (3) is heated to 110.degree. C. to 350.degree. C. at a pressure higher than saturated steam pressure to solubilize the sludge to a state readily degradable by microorganisms. The solubilized sludge is returned to a biological oxidizing tank (2), and the pH value of the sludge in the biological oxidizing tank (2) to 5 to 9. The use of the above method of treating excess sludge and an apparatus therefore enables substantially reducing the volume of excess sludge with economic advantage.