A method for synthesizing a cDNA containing the 5'-terminal sequence of a full-length mRNA having a cap structure from an mRNA sample containing the full-length mRNA having the above cap structure together with a non-full-length mRNA having no cap structure. This method comprises the first step of eliminating the 5'-terminal phosphate group of the non-full-length mRNA in the mRNA sample, the second step of eliminating the 5'-terminal cap structure of the full-length mRNA in the mRNA sample, the third step of bonding an oligonucleotide to the 5'-terminal phosphate group of the mRNA which has been formed in the sample via the first and second steps, and the fourth step of synthesizing the first strand cDNA by subjecting to a treatment with a reverse transcriptase the mRNA carrying the oligonucleotide bonded to the 5'-terminal phosphate group thereof in the third step with the use of a primer comprising a short-chain oligonucleotide annealing with a sequence in the middle of the mRNA.