PURPOSE: To perfectly and efficiently remove ammonia nitrogen and phosphoric acid from waste water at the same time and to remove nitrogen and phosphorus without exerting adverse effect on biological treatment when a biological method is used in combination.

CONSTITUTION: In a method for simultaneously removing nitrogen and phosphorus from waste water by adding a water soluble magnesium compd. to waste water containing ammonia nitrogen and phosphoric acid, ammonia or phosphoric acid is added so that the mol ratio of ammonia nitrogen and phosphoric acid in waste water becomes 1:1.