An objective of the present invention is to facilitate the acquisition of antibody-producing cells that are infiltrating virus-infected cells, cancer cells, abnormal cells forming a benign hyperplasia, and the like, and to improve the efficiency of the production of antibodies as well as nucleic acids encoding them from the antibody-producing cells.The present inventors discovered that, when cancer tissues comprising infiltrating lymphocytes are transplanted into highly immunodeficient animals that do not have T cells, B cells, and NK cells and further exhibit a low IFN production ability, the differentiation and proliferation of infiltrating lymphocytes are unexpectedly promoted, and the number of plasma cells that produce antibodies recognizing cancer tissues increases dramatically, plasma cells can be separated easily, and antibodies or nucleic acids encoding them can be easily prepared from the plasma cells.