It is an object of the present invention to provide a therapeutic agent for cancer comprising an antibody reacting with a cancer-specific membrane antigen wherein the agent is administered by a novel method for decreasing the number of clinical cases for which trastuzumab administration is ineffective. The present invention provides a therapeutic agent for cancer comprising an antibody reacting with a cancer-specific membrane antigen expressed in epithelial cancer cells, said antibody having antibody-dependent cell cytotoxicity, wherein the therapeutic agent for cancer is used for living individual with epithelial cancer which is positive for the cancer-specific membrane antigen and positive for the KLRG1 ligand, and the time of initiation of administration of the therapeutic agent for cancer is during or after the period when KLRG1-positive immunocytes in the peripheral blood of the living individual is selectively reduced ex vivo.
