Plants have a defense system, known as RNA silencing mechanism, against foreign genetic elements, such as viruses. RNA silencing mechanism can efficiently and specifically degrade target RNAs and also works against plant viral vectors and transgenes for recombinant protein production, thereby reducing the yield of recombinant proteins. To improve production level of recombinant proteins, we have repressed or knocked-out RNA silencing mechanism-related genes in Nicotiana benthamiana plants. It was cleared that the repression of RNA silencing mechanism would be useful for mass production of recombinant proteins in plants.