A method for treating ammonia-containing water containing ammonia, metal ions that form a complex with ammonia, and a reducing substance includes a first step of oxidizing the reducing substance by adding hydrogen peroxide to the ammonia-containing water; a second step of aerating the resultant water treated in the first step to remove the ammonia; a third step of forming an insoluble solid at a pH of 7 to 12 by adding at least one compound selected from the group consisting of iron salts, aluminum salts, calcium salts, and calcium hydroxide to the water treated in the second step; and a fourth step of separating the insoluble solid formed in the third step by solid-liquid separation.