To provide a crystallization phosphorus removal method which enables a stable maintenance of a high dephosphorization effect while phosphates are recovered from wastewater containing the phosphates and a low concentration of ammonium ions, and enables a recycle of separated and recovered crystals as a fertilizer or a soil conditioner.

In a phosphate-containing wastewater treatment method for removing the phosphates from phosphate-containing wastewater as magnesium phosphate (Mg3(PO4)2xH20) in presence of magnesium ions, magnesium ammonium phosphate (MgNH4PO4 6H20) is filled as a seed crystal in a crystallization reaction tank crystallizing magnesium phosphate crystals, and then made to flow after adding a magnesium agent and an alkali agent thereto, thereby crystallizing and recovering the magnesium phosphate crystals.