To provide a method for treating water containing phosphorus, by which a reaction probability is increased without using a special catalyst to increase a reaction rate and water can be economically treated in an industrial scale.

A chemical which generates hydroxyl radicals is dissolved in water containing phosphorus to generate hydroxyl radicals. By the homogeneous liquid phase oxidation treatment of the hydroxyl radicals and the phosphorus in the water containing phosphorus, the phosphorus is oxidized to produce orthophosphate ions so as to separate and remove phosphorus.