To improve detection accuracy, quantitativity and the like, when a double strand nucleic acid is detected with an intercalator.

The inventors of the present invention have newly found that an intercalator interacting with a site continuously having AT base pairs and with a site having one GC base pair or continuously having GC base pairs in a double strand nucleic acid does not cause a concentration quenching until to a prescribed amount, even when added in a more excessive amount than an addition amount for a general intercalator, and can increase fluorescence intensity by the addition of the intercalator in a more excessive amount than an addition amount for a conventional intercalator. This method for detecting the double strand nucleic acid with the intercalator is characterized by charging the intercalator in a reaction field in an amount which is more excessive than the saturated interaction amount of the intercalator for the double strand nucleic acid and does cause a concentration quenching.

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