PROBLEM TO BE SOLVED: To provide a method for treating ammonia-nitrogen-containing water where the concentration of ammonia-nitrogen in water can be stably reduced.SOLUTION: Provided is a method for treating ammonia-nitrogen-containing water characterized in that ammonia-nitrogen-containing water containing either or both of methane and hydrogen sulfide and ammonia-nitrogen-containing water is subjected to nitration treatment by a catalytic oxidation method utilizing nitration bacteria stuck to a bacteria carrier without using a nitration denitrification method, and is also exposed to the exposure strength of 2 to 10 m/mhr. As the bacteria carrier, a string-like carrier 10 made of a string-like body having a branched dendritic structure from a stem 12 to a ring-shaped branch part 14, and in which the diameter of the stem part 12 is thicker than that of the ring-shaped branch part 14.
