A method for detecting a miRNA molecule, comprising: (a) reverse-transcribing a miRNA molecule using a reverse transcriptase and a short reverse primer for reverse transcription composed of a first hybridizing oligonucleotide and a first adaptor oligonucleotide, the first hybridizing oligonucleotide being composed of a nucleic acid sequence which hybridizes to a 3'-end portion of the miRNA molecule having 5 to 9 nt at the 3'-end of the miRNA molecule, and the first adaptor oligonucleotide being attached to the 5'-end of the first hybridizing oligonucleotide and being any nucleic acid sequence which does not hybridize to the miRNA molecule; (b) deactivating the reverse transcriptase and melting a DNA produced by the reverse transcription, thereby preparing a single-stranded cDNA reverse-transcribed from the miRNA molecule; and (c) transferring a portion or all of the reverse-transcription reactant into a reactor for primer extension and PCR reactions containing an extension primer longer than the reverse primer for reverse transcription, a forward primer, and a heat-stable DNA polymerase, the extension primer being composed of a second hybridizing oligonucleotide and a second adaptor oligonucleotide, the second hybridizing oligonucleotide being capable of hybridizing specifically to a portion of the single-stranded cDNA reverse-transcribed from the miRNA molecule, the portion of the single-stranded cDNA excluding a portion corresponding to the 3'-end portion of the miRNA molecule, and the second adaptor oligonucleotide being attached to the 5'-end of the second hybridizing oligonucleotide and having any nucleic acid sequence which does not hybridize to the single-stranded cDNA, and the forward primer having a sequence within the second adaptor oligonucleotide, and then performing primer extension and PCR reactions in a single step; wherein a reverse primer is not added in the (c); wherein the short reverse primer for reverse transcription used in the reverse transcription of the (a) also functions as a reverse primer in the PCR reaction, and thus the short reverse primer for reverse transcription contained in the reverse transcription reactant is used as a reverse primer in the PCR reaction; wherein the short reverse transcription primer is 18 to 28 nt in length; and wherein the second adaptor oligonucleotide is 56 to 63 nt in length. (a) reverse-transcribing a miRNA molecule using a reverse transcriptase and a short reverse primer for reverse transcription composed of a first hybridizing oligonucleotide and a first adaptor oligonucleotide, the first hybridizing oligonucleotide being composed of a nucleic acid sequence which hybridizes to a 3'-end portion of the miRNA molecule having 5 to 9 nt at the 3'-end of the miRNA molecule, and the first adaptor oligonucleotide being attached to the 5'-end of the first hybridizing oligonucleotide and being any nucleic acid sequence which does not hybridize to the miRNA molecule; (b) deactivating the reverse transcriptase and melting a DNA produced by the reverse transcription, thereby preparing a single-stranded cDNA reverse-transcribed from the miRNA molecule; and (c) transferring a portion or all of the reverse-transcription reactant into a reactor for primer extension and PCR reactions containing an extension primer longer than the reverse primer for reverse transcription, a forward primer, and a heat-stable DNA polymerase, the extension primer being composed of a second hybridizing oligonucleotide and a second adaptor oligonucleotide, the second hybridizing oligonucleotide being capable of hybridizing specifically to a portion of the single-stranded cDNA reverse-transcribed from the miRNA molecule, the portion of the single-stranded cDNA excluding a portion corresponding to the 3'-end portion of the miRNA molecule, and the second adaptor oligonucleotide being attached to the 5'-end of the second hybridizing oligonucleotide and having any nucleic acid sequence which does not hybridize to the single-stranded cDNA, and the forward primer having a sequence within the second adaptor oligonucleotide, and then performing primer extension and PCR reactions in a single step; wherein the short reverse primer for reverse transcription used in the reverse transcription of the (a) also functions as a reverse primer in the PCR reaction, and thus the short reverse primer for reverse transcription contained in the reverse transcription reactant is used as a reverse primer in the PCR reaction; wherein the short reverse transcription primer is 18 to 28 nt in length; and wherein the second adaptor oligonucleotide is 56 to 63 nt in length.