To provide a method of measurement for phosphoric acid in water capable of continuously and accurately measuring concentration of inorganic phosphoric acid in sample water on real time, free from affection of other substance, and to provide equipment for the same.

The method measures the concentration of inorganic phosphoric acid in the sample water 12 by mixing a reagent 13 including glyceraldehyde-3- phosphate (GAP) and nicotinamide adenine dinucleotide (NAD+) which is a coenzyme, with the sample water 12, and producing glyceric acid-1,3-diphosphoric acid (GDP) and reduced nicotinamide adenine dinucleotide (NADH) in the presence of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as a catalyst in a reactor 1, and optically detects the produced amount of the NADH by change of optical absorption at 340 nm of flow cells 6a, 6b installed on before and after the reactor.