The disclosure relates to a method of introducing mRNA encoding Cas9 protein ( Cas9 mRNA) into a mammalian embryo, comprising the steps of； (a) placing a mixture of the mammalian embryo and a solution comprising Cas9 mRNA in the gap between a pair of electrodes, and (b) applying a voltage to the electrodes for a voltage application duration, wherein the voltage and the voltage application duration achieve the efficiency of mRNA introduction (R) higher than the minimum required efficiency of mRNA introduction (R min ) that is calculated on the basis of the concentration of Cas9 mRNA (ng/µl).
