To obtain a nucleic acid stain useful for gene diagnosis by including a specific hydroxynaphthylazohydroxynaphthalenesulfonic acid salt (derivative) so as to interact with a double-stranded nucleic acid to effect change of fluorescence intensity and fluorescence wavelength and improve assay accuracy.

This nucleic acid stain comprises a 4-(2-hydroxy-1-naphthylazo)-3- hydroxy-1-naphthalenesulfonic acid salt (derivative) of the formula (R1 is OH, an alkyl, phenyl, amino or anilino; R2 is SO3Na, SO3K or SO3Li). It is preferable that a double-stranded nucleic acid is detected by measuring the change of the fluorescence properties caused by the interaction of this compound with the double-stranded nucleic acid, and it is also preferable that the other objective detection reagent for a targeted nucleic acid is prepared by including the compound of the formula and a probe consisting of a single-stranded nucleotide having a nucleic acid sequence complementary to the targeted nucleic acid.