An object of the present invention is to provide a method of conveniently producing a genetically modified non-human mammal with high efficiency using a CRISPR-Cas9 system and particularly a production method whereby gene knock-in can be achieved with high efficiency regardless of the gene size. The method of producing a genetically modified non-human mammal comprises introducing a Cas9 protein, a crRNA fragment comprising a nucleotide sequence complementary to a target DNA region, and a tracrRNA fragment into a non-human mammalian oocyte to genetically modify the target DNA.
