A method for assaying a target nucleic acid, comprising: providing an RNA amplification system comprising producing a double-stranded DNA using, as a template, a target RNA containing a specific nucleotide sequence in a sample, said double-stranded DNA having a promoter sequence and being capable of transcribing an RNA comprising the specific nucleotide sequence or a sequence complementary to the specific nucleotide sequence, producing an RNA transcription product comprising the specific nucleotide sequence or a sequence complementary to the specific nucleotide sequence in the presence of an RNA polymerase, and producing the double-stranded DNA using the RNA transcription product as a template, in the presence of a probe labeled with an intercalating fluorochrome having a sequence complementary to the RNA transcription product; measuring the fluorescence intensity in the RNA amplification system with time; calculating a time when the fluorescence intensity satisfies a prescribed criterion based on the measured change in the fluorescence intensity with time; and determining a concentration of the target nucleic acid in the sample based on the calculated time.