PURPOSE: To provide a method for simultaneously measuring concentrations of organic acid, ammonia and phosphorus of high concentrations of digested sludge in a sample containing the three items.

CONSTITUTION: A sample to be measured and distilled water as a blank sample are prepared. Phosphate ion insolubilizer is added to both the samples, then solid-liquid separated, separated liquid is acidified, then soluble carbonic acid is volatilized, and titrated corresponding to a predetermined titrating pH range with alkaline solution. Then, overall organic acid concentration and overall amonia concentration are calculated by comparison of its titer with blank test, precipitate after the solid-liquid separation is dissolved with strong acid, dissolved carbonic acid is volatilized, and then similar alkaline titration is conducted by using the alkaline solution. Concentration of phosphoric acid ion state phosphorus in the sample to be measured is calculated by a comparison of the titer with the blank test.