An object of the present invention is to produce a mammalian organ having a complicated cellular composition composed of multiple kinds of cells, such as kidney, pancreas, thymus and hair, in the living body of a non-human animal. The inventors of the present invention applied the chimeric animal assay described above, to a novel solid organ production method. More specifically, the inventors has shown that a model mouse which is deficient of kidney, pancreas, thymus or hair due to the dysfunction of the metanephric mesenchyme that is differentiated into most of an adult kidney, is rescued by blastocyst complementation by the chimeric animal assay, and whereby a kidney, a pancreas, thymus or hair can be newly produced.
