The purpose of the present invention is to produce an antibody having higher antitumor activity than conventional antibodies that target CAPRIN-1, which is specifically expressed on the surface of a cancer cell, and to provide the use of the antibody as an agent for the treatment and/or prophylaxis of cancer. The present invention provides the use, as an agent for the treatment and/or prophylaxis of cancer, of an antibody that identifies and targets a cancer antigen protein that is specifically expressed on the surface of a cancer cell. More specifically, the present invention provides a pharmaceutical composition for the treatment and/or prophylaxis of cancer, the pharmaceutical composition being characterized by containing, as an active ingredient: an antibody that has immunological reactivity with CAPRIN-1 protein and that comprises a heavy-chain variable region including amino acid sequences represented by SEQ ID NOs: 5, 6, and 7 and a light-chain variable region including amino acid sequences represented by SEQ ID NOs: 9, 10, and 11 or a fragment of said antibody.
