The method for treating an ammonium-containing liquid which biologically denitrifies ammonium contained in the ammonium-containing liquid in an anaerobic atmosphere, the method comprises the steps of: bringing the ammonium-containing liquid into contact with anaerobic bacteria in a biological treatment tank, the anaerobic bacteria having an ability to biologically denitrify the ammonium in the anaerobic atmosphere; and adding nitrite to the biological treatment tank from a nitrite storage tank concurrently with the bringing step, the nitrite storage tank storing the nitrite with a certain concentration.