To provide a method for evaluating in a simple manner the phosphorus-containing crystallized substance recovered from a crystallizing dephosphorization apparatus as purity by general-purpose evaluation interlocked with on-the-spot operation control.

After an acid is added to the phosphorus-containing crystallized substance, such as, ammonium magnesium phosphate, magnesium phosphate, magnesium hydrogen phosphate, hydroxyapatite, recovered from wastewater or sludge by a crystallizing dephosphorization method and the phosphorus-containing crystallized substance is heated; a supernatant liquid is obtained by the solid-liquid separation of the phosphorus-containing crystallized substance, to which the acid is added; and absorbance of the supernatant liquid at a predetermined wavelength is subsequently measured to determine the purity of the phosphorus-containing crystallized substance.