A method utilizing inorganic nutrient salts, which are inexpensive and easily usable, for waste water whose aerobic and/or anaerobic biological treatment cannot be established unless inorganic nutrient salts are added, is provided. A method and apparatus are provided for utilizing recovered magnesium ammonium phosphate, characterized by adding the recovered magnesium ammonium phosphate, which has been recovered from a treatment process for organic waste and/or organic waste water treatment, to a biological treatment step comprising an anaerobic treatment step and/or an aerobic treatment step to utilize the recovered magnesium ammonium phosphate as an inorganic nutrient source. Preferably, the recovered magnesium ammonium phosphate is utilized as particles, with its particle size being 0.5 mm or less, and/or the pH of a liquid, to which magnesium ammonium phosphate is added, being 10 or lower.