The present inventors discovered that genes could be introduced specifically into trophectodermal cells with high efficiency, by infecting blastocysts with viral vectors carrying an arbitrary polynucleotide, or by using a nucleic acid transfection reagent in blastocysts, from which zona pellucida (extracellular matrix covering preimplantation early embryos to protect them from infection of viruses and the like) is removed. This method has no risk of infecting cells of the inner cell mass, which develops into a fetus in the future, with the introduced polynucleotide because the trophectoderm serves as a barrier. The present invention provides methods for introducing foreign genes into only placenta but not fetus, which enables rescue of genetically mutant animals from embryonic lethality due to placental abnormality and allows their birth. Furthermore, it is possible to analyze expression and effect of genes that regulate placental formation or placental function by using these methods.