A simple and accurate method for assay of a single-stranded RNA containing a specific nucleic acids sequence in a sample at almost constant temperature by using at least the following reagents (A) to (I), which comprises a step of adding the reagents (A) to (I) one by one (in any order), in combinations of at least two or all at once and a step of measuring a fluorescent signal in the presence of the reagent (I) at least once after addition of at least the reagents (A) to (H); (A) a first single-stranded oligonucleic acid complementary to a sequence neighboring the 5′ end of the specific nucleic acids sequence in the single-stranded RNA, (B) a second single-stranded oligo DNA complementary to a 3′-end sequence within the specific nucleic acids sequence, (C) an RNA-dependent DNA polymerase, (D) deoxyribonucleoside triphosphates, (E) a third single-stranded oligo DNA having (1) a promoter sequence for a DNA-dependent RNA polymerase, (2) an enhancer sequence for the promoter and (3) a 5′-end sequence within the specific nucleic acids sequence, in this order from the 5′ end, (F) a DNA-dependent DNA polymerase, (G) a DNA-dependent RNA polymerase,