An object is to develop a technology enabling utilization, by only an extremely simple technique, of a technology which is widely applicable to mammals without requiring the utilization of an ES cell, and which involves modifying a certain gene by targeting a certain sequence on a genome (genome editing technology based on ZFN or the like). Provided is a technology for efficiently modifying an arbitrary target gene of a mammal, by immersing a pronuclear stage mammalian zygote with an intact zona pellucida into a solution containing a pair of molecules of mRNA having a certain sequence, and performing electroporation treatment through application of multiple square-wave pulses in three steps with the total electric energy of a first electric pulse adjusted within a predetermined range.
