Experiments were conducted to elucidate the auxiliary effect of adding fibrous materials recovered from a primary settling tank on dewatering digested sludge. Experiments with a constant-pressure filtration device revealed that addition of the fibrous materials increased the filtration rate and decreased the average specific filtration resistance. This improvement in filterability increased with the increase in the amount of the fibrous materials added. Experiments with a full-scale screw press for dewatering demonstrated a decrease in the water content of the dewatered cake from 82 to 68% on addition of the fibrous materials. The correlation between the increase in the proportion of fibrous material added and the decrease in water content of the dewatered cake could be rationally described by a quadratic equation, indicating that the water content of dewatered cake can be controlled by adjusting the proportion of fibrous materials added. Furthermore, the addition of the fibrous materials to digested sludge increased the speed of treatment and decreased the amount of polymer coagulant required. These findings indicate that fibrous materials recovered from a primary settling tank are promising auxiliary agents to improve the dewatering of digested sludge.