A novel technique of immobilizing nitrifying bacteria using fibrous material, the three-dimensional immobilization with a fibrous network (3-D IFN), is proposed and its use in a high-rate nitrifying bioreactor is investigated. The fibrous carrier employed is ferro-nickel fibrous slag (FS), which is industrial solid waste from a ferro-nickel electrosmelting process. Since cell immobilization with FS is readily carried out within 90 seconds, this method is by far more rapid than any other cell immobilization techniques. A fixed-bed nitrifying reactor packed with cell-immobilizing FS is examined by both batch-mode and continuous feeding tests. By controlling the circulation flow rate, the transport of dissolved oxygen to the immobilized cells is improved, which is a distinct characteristic of this reactor. The continuous feeding test revealed that the ammonia removal rate of the reactor reached 6.5 kg-N/(m3-reactor)/d, which was extremely high compared with that of the conventional fixed-bed reactor for wastewater treatment. It is demonstrated that the 3-D IFN is a simple yet effective cell immobilization technique that can be successfully used in a high-rate nitrifying reactor.