PURPOSE: To provide the phosphorus-concentration measuring method, by which measuring errors are reduced to the minimum degree by measuring samples without dillution and the measuring time can be shortened, when the phosphorus in high concentration such as in excrement and mud is measured.

CONSTITUTION: Reagent wherein phosphoric-acid ion is not dissolved is added into a specimen sampled from mud or excrement, and phosphorus is sedimented. The sediment is dissolved with a strong acid. Water is added, and a sample solution for titration is obtained. An alkali solution is used for the sample solution, and the titration is performed. The concentration of the phosphorus in the phosphoric-acid state in the sample solution is computed by using computing expression basing on the amount of the alkali solution required for the titration. As the pretreatment of the specimen, the specimen is applied into a centrifugal separator. The supernatant liquid in the centrifugal separator is sampled, and the sample solution is obtained. Or the specimen is applied into an ultrafilter. The filtered liquid through the ultrafilter film is sampled, and the sample solution is obtained.