PROBLEM TO BE SOLVED: To provide a method for sanitarily preserving and processing organic sludge at a low equipment cost.SOLUTION: Organic sludge S is mixed with one or more kinds of ventilation improving material C selected from the group consisting of wood trash, fiber trash, rubber trash, paper trash, waste plastic, waste tire, waste PET bottles, plastic container packing, expanded polystyrene, rice straw, sawdust, chaff, bark, perlite, vermiculite, lightweight aggregate, and waste ALC. The organic sludge S is further mixed with waste activated carbon W. Stacked organic sludge P is then kept at a temperature of 60°C or lower by ventilation.SELECTED DRAWING: Figure 1