Very simple, highly sensitive detection or quantification of target nucleic acids of interest has been achieved by: hybridizing mask oligonucleotides to regions in a single-stranded region of a nucleic acid to be assayed between which a region to be hybridized by an oligonucleotide probe is positioned, thereby opening the probe-hybridizing region and keeping the single-stranded region of the target nucleic acid stable, and then subjecting this nucleic acid having the single-stranded region to nucleic acid chromatography.