In vitro synthesized mRNAs are often used as calibrants in gene expression analysis. In the case of an analytical procedure for gene expression containing a process of reverse transcription such as microarray analysis, full-length mRNAs having a 3'-poly(A)+tail are required as calibrants. However, the preparation of full-length mRNAs having a 3'-poly(A)+tail by using ordinary plasmid vectors is difficult. In this study, we established two plasmid vectors in which the nucleotide sequence corresponding to the 3'-poly(A)+tail were included. By simply inserting full-length cDNAs lacking their 3'-poly(A)+tail into established plasmid vectors, full-length mRNAs having a 3'-poly(A)+tail were successfully prepared.