According to the present invention, an inorganic phosphoric acid is detected by a method which includes: subjecting a sample to a measurement system containing glyceraldehyde-3-phosphate, oxidized nicotinamide adenine dinucleotide or oxidized nicotinamide adenine dinucleotide phosphate, glyceraldehyde phosphate dehydrogenase, and an electron mediator; and measuring a current value in the measurement system. In the method, a pyrophosphate is quantitatively measured with high sensitivity and at a high speed through converting the pyrophosphate in a sample into an inorganic phosphoric acid. Such a measurement of a pyrophosphate allows for quantitative determination of the pyrophosphate which is produced concurrent with the extension of a DNA, thereby enabling the detection of the presence of a targeted nucleic acid, and typing of a base in a SNP site of a targeted DNA.