Abstract: The invention offers a method and apparatus for solubilizing excess sludge, having a high solubilizing ability, a low total cost, and capable of reducing the size of facilities. The method for reducing excess sludge comprises adding a solubilizing agent to excess sludge generated by microbial treatment of organic sewage, applying ultrasonic waves, applying a reduced pressure swelling step, then returning the result to the microbial treatment, whereby the volume of the excess sludge can be reduced. The apparatus for reducing excess sludge comprises means for adding a solubilizing agent to excess sludge generated by applying a microbial treatment to organic sewage, ultrasonic wave applying means for applying ultrasonic waves, and reduced pressure swelling means provided downstream of the ultrasonic wave applying means for applying a reduced pressure swelling process to the excess sludge.