A method of manufacturing a mixture of amplified double-stranded nucleic acids comprising: (a) preparing a single-stranded nucleic acid comprising a single-stranded adapter 1, a single-stranded nucleic acid fragment and a single-stranded adapter 2, wherein said single-stranded adapter 1 comprises at least a common sequence 1 and a suffix sequence 1, said single-stranded adapter 2 comprises at least a suffix sequence 2 and a common sequence 2, wherein said common sequence 1 and said common sequence 2 are reverse-complementary to each other, and said suffix sequence 1 and said suffix sequence 2 are not reverse-complementary to each other; wherein the single-stranded adapter 1 is one comprising the common sequence 1 and the suffix sequence 1 in 5' to 3' direction, and wherein the single-stranded adapter 2 is one comprising the suffix sequence 2 and the common sequence 2 in 5' to 3' direction, wherein the common sequence 1 and the common sequence 2 have the same length of the nucleotide sequence which is in a range of from 10 to 40 nucleotides; wherein said step (a) comprises nucleic acid strand synthesis reaction using single-stranded template nucleic acid comprising a sequence complementary to the single-stranded nucleic acid fragment and having a cap structure at 5' end thereof; an oligonucleotide comprising at the 3' end thereof nucleotides that can hybridize to extra 3'-nucleotides at the 3' end of the reverse-transcribed template, which is the first strand cDNA, that is complementary to the single-stranded template nucleic acid, and a sequence reverse-complementary to the adapter 2; and a primer 3 comprising at least a random sequence or an oligo-T at 3' end thereof and a sequence corresponding to the adapter 1. and (b) conducting PCR with said single-stranded nucleic acid prepared in step (a), a primer 1 comprising at least a part of the common sequence 1 and the suffix sequence 1, and a primer 2 comprising a part of the reverse-complement of the common sequence 2 and the reverse-complement of the suffix sequence 2 to amplify double-stranded nucleic acids, wherein the PCR is carried out under conditions where intramolecular folding takes place. (a) preparing a single-stranded nucleic acid comprising a single-stranded adapter 1, a single-stranded nucleic acid fragment and a single-stranded adapter 2, wherein said single-stranded adapter 1 comprises at least a common sequence 1 and a suffix sequence 1, said single-stranded adapter 2 comprises at least a suffix sequence 2 and a common sequence 2, wherein said common sequence 1 and said common sequence 2 are reverse-complementary to each other, and said suffix sequence 1 and said suffix sequence 2 are not reverse-complementary to each other; wherein the single-stranded adapter 1 is one comprising the common sequence 1 and the suffix sequence 1 in 5' to 3' direction, and wherein the single-stranded adapter 2 is one comprising the suffix sequence 2 and the common sequence 2 in 5' to 3' direction, wherein the common sequence 1 and the common sequence 2 have the same length of the nucleotide sequence which is in a range of from 10 to 40 nucleotides; wherein said step (a) comprises nucleic acid strand synthesis reaction using single-stranded template nucleic acid comprising a sequence complementary to the single-stranded nucleic acid fragment and having a cap structure at 5' end thereof; an oligonucleotide comprising at the 3' end thereof nucleotides that can hybridize to extra 3'-nucleotides at the 3' end of the reverse-transcribed template, which is the first strand cDNA, that is complementary to the single-stranded template nucleic acid, and a sequence reverse-complementary to the adapter 2; and a primer 3 comprising at least a random sequence or an oligo-T at 3' end thereof and a sequence corresponding to the adapter 1. and (b) conducting PCR with said single-stranded nucleic acid prepared in step (a), a primer 1 comprising at least a part of the common sequence 1 and the suffix sequence 1, and a primer 2 comprising a part of the reverse-complement of the common sequence 2 and the reverse-complement of the suffix sequence 2 to amplify double-stranded nucleic acids,