A method for producing a model animal which has a desired lifetime, and in which a predetermined biological reaction can be induced, and a model animal are provided. The present invention produces a first individual in which a gene of interest is heterozygously deficient using a first ES cell from a non-human mammalian animal. Meanwhile, a fragment containing a homologous gene that has homology to the gene of interest is made, a second ES cell constituted so that a predetermined region on X chromosome of the animal can be substituted is used, and the fragment is introduced into the second ES cell to generate a substituted ES cell in which the predetermined region has been substituted with the fragment. A second individual is produced using the substituted ES cell. The first individual and the second individual are mated with each other to produce a model animal.