A non-human mammalian gene modification method, comprising: immersing a non-human zygote as defined in the following item into a solution containing a nucleic acid molecule as defined in the following item; applying a square-wave electric pulse as defined in the following item to the solution once or two or more times so that the square-wave electric pulse has a total electric energy of from 0.2 J/100 μL to 7.5 J/100 μL; then applying a square-wave electric pulse as defined in the following item two or more times; and then applying a square-wave electric pulse as defined in the following item two or more times: a pronuclear stage zygote of a mammal, except of a human, with an intact zona pellucida; RNA which functions so as to exhibit endonuclease activity against an arbitrary region of genomic DNA in a sequence-specific manner; a square-wave electric pulse having a voltage per pulse of 375 V/cm or more; a square-wave electric pulse having a voltage per pulse of 250 V/cm or less and an electric energy per pulse of from 0.01 J/100 μL to 3.6 J/100 μL; and a square-wave electric pulse which is opposite in polarity to the electric pulse as defined in the item and has a voltage per pulse of 250 V/cm or less and an electric energy per pulse of from 0.01 J/100 μL to 3.6 J/100 μL. immersing a non-human zygote as defined in the following item into a solution containing a nucleic acid molecule as defined in the following item; applying a square-wave electric pulse as defined in the following item to the solution once or two or more times so that the square-wave electric pulse has a total electric energy of from 0.2 J/100 μL to 7.5 J/100 μL; then applying a square-wave electric pulse as defined in the following item two or more times; and then applying a square-wave electric pulse as defined in the following item two or more times: a pronuclear stage zygote of a mammal, except of a human, with an intact zona pellucida; RNA which functions so as to exhibit endonuclease activity against an arbitrary region of genomic DNA in a sequence-specific manner; a square-wave electric pulse having a voltage per pulse of 375 V/cm or more; a square-wave electric pulse having a voltage per pulse of 250 V/cm or less and an electric energy per pulse of from 0.01 J/100 μL to 3.6 J/100 μL; and a square-wave electric pulse which is opposite in polarity to the electric pulse as defined in the item and has a voltage per pulse of 250 V/cm or less and an electric energy per pulse of from 0.01 J/100 μL to 3.6 J/100 μL. a pronuclear stage zygote of a mammal, except of a human, with an intact zona pellucida; RNA which functions so as to exhibit endonuclease activity against an arbitrary region of genomic DNA in a sequence-specific manner; a square-wave electric pulse having a voltage per pulse of 375 V/cm or more; a square-wave electric pulse having a voltage per pulse of 250 V/cm or less and an electric energy per pulse of from 0.01 J/100 μL to 3.6 J/100 μL; and a square-wave electric pulse which is opposite in polarity to the electric pulse as defined in the item and has a voltage per pulse of 250 V/cm or less and an electric energy per pulse of from 0.01 J/100 μL to 3.6 J/100 μL.