There is provided a method for quantifying a subject substance, of which typical examples are amino acids. The method of the present invention comprises the following steps: the step of allowing an enzyme that can generate pyrophosphate by using adenosine triphosphate (ATP) as a substrate with converting the subject substance to act on the subject substance to generate pyrophosphate; the step of allowing pyruvate pyrophosphate dikinase (PPDK) to act on the generated pyrophosphate in the presence of adenosine monophosphate (AMP) and phosphoenolpyruvate (PEP) to generate ATP, phosphoric acid, and pyruvate; and the step of quantifying the generated pyruvate, and amount of the subject substance is determined on the basis of the obtained amount of pyruvate. According to the present invention, an amino acid in a biological sample containing a lot of various kinds of contaminants such as inorganic phosphoric acid and urea can be conveniently and quickly quantified without being influenced by the contaminants.
