To provide a method for cultivating ammonia oxidative bacteria by which the proliferation concentration of the ammonia oxidative bacteria of species adaptive to low concentration of the ammonia is improved, the stable proliferation is enabled, and the cultivation period is shortened by adding heterotrophic bacteria to the cultivation liquid when cultivating the ammonia oxidative bacteria.

This method for cultivating substrate-specific ammonia oxidative bacteria whose proliferation is inhibited by highly concentrated ammonia, and having high compatibility with the low concentration ammonia comprises adding heterotrophic bacteria (e.g. bacteria of Paracoccus and Pseudomonas), a culturing liquid thereof or a filtrate of the culturing liquid thereof to the cultivating liquid of the ammonia oxidative bacteria. The heterotrophic bacteria or the culturing liquid thereof is preferably added so that the initial concentration of the heterotrophic bacteria may be 10-104 cells/mL. In the case of the filtrate of the culturing liquid of the heterotrophic bacteria, the proportion of the filtrate of the culturing liquid is preferably regulated so as to be 0.5-50 vol.% in the cultivation medium of the ammonia oxidative bacteria after the addition.