Abstract: A process for treating sludge is disclosed. Sludge to which a flocculating agent has been added is stirred by a stirring pump or shear-stirred by a liquid shear-stirrer prior to initiation of flocculation reaction of the flocculating agent to render the flocculating agent in a fine particulate state to thereby disperse, diffuse or distribute the flocculating agent in a fine particulate state throughout the sludge, so that flocks or aggregate structure type flocks or blocks can be efficiently formed. A treating line is constructed such that the sludge can flow therein as a laminar flow. The distance from a flocculating agent injection part to a stirring pump is made to a distance such that the flocculating agent can pass through in a reaction initiation time obtained in advance for the flocculating agent.