The present invention provides an adult T cell leukemia model animal, which is a T cell function deficient animal to which a cell line infected with human T cell leukemia virus-1 is transplanted. This model animal allows the HTLV-1 infected cell line to proliferate over a long period of time, and enables not only the tumorigenesis process but also the mechanism of onset of ATL and immune response mechanism of the host against ATL to be precisely analyzed.