A process for treating ammonia containing wastewater includes bringing an ammonia-treating material and ammonia containing wastewater into contact with each other to remove ammonia in the wastewater continuously as nitrogen gas, the ammonia-treating material including a long carrier and complex bacterial sludge attached and immobilized on the biomass carrier, the carrier including a net, a nonwoven fabric or a woven fabric of fibers or filaments, the carrier being attached to a support, the complex bacterial sludge containing bacterial sludge including autotrophic anammox bacteria and bacterial sludge including autotrophic ammonia-oxidizing bacteria, the ammonia containing wastewater containing dissolved oxygen at a concentration of not less than 0.5 mg/l. The process for treating ammonia containing wastewater uses the treating material in which the bacterial sludge are attached and immobilized, and nitritation and anammox reaction can take place efficiently and economically even when the wastewater contains dissolved oxygen at a high concentration.