A method of amplifying nucleic acids which involves the following steps: (I) the step of forming a single-stranded DNA complementary to an mRNA in a sample with a first oligonucleotide primer; (II) the step of synthesizing a DNA strand complementary to the single-stranded DNA obtained in the step (I) to thereby form a double-stranded DNA; (III) the step of adding a single- or double-stranded adapter DNA to the double-stranded DNA obtained in the step (II) to give a DNA strand; and (IV) the step of amplifying the DNA strand obtained in the step (III) with the use of a second oligonucleotide primer which is an oligonucleotide having a nucleic acid sequence contained in the adapter DNA, and by which nucleic acids having base sequences of mRNAs contained in a sample can be amplified while sustaining the ratio among these mRNAs.
