A method for amplifying a trace amount of mRNA includes adding a dummy RNA to a solution containing the trace amount of mRNA, so as to prepare a mixed solution; synthesizing an anti-sense DNA by reverse transcription, which uses the mixed solution as a template; synthesizing a sense DNA which is complementary to the anti-sense DNA thus synthesized, so as to generate a double-strand DNA made of the sense DNA and the anti-sense DNA; ligating an RNA polymerase promoter sequence to the double-strand DNA thus generated, on a sense DNA 5′ end side of the double-strand DNA, so as to prepare a double-strand DNA for amplification; and amplifying, by using RNA polymerase, an RNA from the double-strand DNA for amplification.