An object of the present invention is to provide a mouse that enables the functions of human NK cell to be studied. A DNA consisting of a nucleotide sequence represented by SEQ ID NO: 1, which is a gene region comprising a DNA in which a cDNA sequence encoding interleukin 15 (IL-15) is operably ligated to a cDNA sequence encoding the signal peptide of human interleukin (IL-2), is inserted to immunodeficient mouse cDNA. In NOD-scid, IL-2rγnull-hIL-15 Tg mice thus generated, hCD56+ cell having a concentration sufficient for conducting in vivo study on human mature NK cell are detected for at least 6 months after transplantation.
