To provide a method for reducing the volume of sludge which has cost-effectiveness, hardly imposes a load on the environment and exhibits excellent efficiency when used for reducing the volume of sludge.

The method for reducing the volume of sludge comprises the steps of: adding an oxidizing agent based on peracetic acid to a pipeline 7 by using an oxidizing agent adding means 2 when the sludge received in a sludge storage tank 1 is introduced into a sludge solubilization tank 4; solubilizing the sludge by an oxidative action of the oxidizing agent in the sludge solubilization tank; sending the solubilized sludge to a solid-liquid separation tank 5 through a pipeline 8 without neutralizing the solubilized sludge showing acidity; and decomposing the organic matter contained in the sent sludge by using a microbe and separating into the organic matter-decomposed sludge and a supernatant liquid. According to this reducing method, the volume of sludge can be reduced economically while hardly imposing the load on the environment.