To provide a method for treating methane-fermentation digestive-fluid of livestock excretions which reduces the investment and running costs of a post-stage treating device by reducing the amount of a filtrate in the post-stage treatment by decreasing the amount of the filtrate discharged after solid-liquid separation of the digestive fluid.

In the method for treating the digestive fluid for solid-liquid separation of a solid from the filtrate in a solid-liquid separator 2 after an organic polymer coagulant together with a dilution liquid for the coagulant is added to the methane-fermentation digestive-fluid of the livestock excretions from a coagulant dilution vessel 1, a part of the filtrate as a dilution liquid for the coagulant is effectively utilized by refluxing it through a filtrate reflux line (6) to the vessel 1.