A technique is needed which can amplify NK cells in vitro and prepare optimum number of NK cells for the adoptive immunotherapy. A method for amplifying NK cells is provided which comprises steps of: preparing cell population which is comprised of NK cells, removing T cells from the cell population which is comprised of NK cells, and, after removal of T cells, cultivating the remaining cells in a medium supplemented with 2500 to 2831 IU/mL of IL-2. The method for amplifying NK cells of the present invention may comprise a step of removing hematopoietic progenitor cells from the cell population. The present invention provides a pharmaceutical composition for adoptive immunotherapy, comprising NK cells which are prepared by the amplifying method of the present invention.
