The present invention provides a retroviral vector containing a transcription unit comprising a transcription regulatory sequence and a gene encoding a polypeptide having single-stranded RNA-specific endoribonuclease activity which is placed so that its expression can be controlled by the regulatory sequence, wherein the unit is placed so that the direction of the transcription of mRNA from the unit is opposite to the direction of transcription of the RNA genome of the retroviral vector. By using the vector constructed as described above, viral supernatant showing high gene transfer efficiency can be prepared. The retroviral vector of the present invention is useful for the treatment and/or prevention of cancers and virus infections.
