The purpose of the subject invention is to provide a method of manufacturing a mixture of amplified double-stranded nucleic acids comprising unknown sequence including the complete 5′ end sequence.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, and (b) conducting PCR with said single-stranded nucleic acid prepared in step (a), a primer 1, and a primer 2 to amplify double-stranded nucleic acids.