The invention provides a method of preparing a nucleic acid population suitable for RNA sequencing. The method involves amplifying a double-stranded DNA and a poly T sequence by using the DNA constituted of any additional nucleic acid sequence X, poly T sequence, mRNA sequence isolated from a biological sample, poly A sequence and any additional nucleic acid sequence Y in this order as a template, a first primer containing any additional nucleic acid sequence X having amine added to the 5′-terminal (and a poly T sequence), and a second primer containing any additional nucleic acid sequence Y (and a poly T sequence), followed by fractionalizing the DNA, phosphorylating the DNA, preparing cDNA by using the DNA as a template and a third primer, adding adenine (A) to the cDNA, linking a DNA, and amplifying the DNA by using the DNA as a template, a fourth primer, and a fifth primer.