The objective of the present invention is to produce an antibody having superior anti-tumor activity compared to conventional antibodies and having, as a target, CAPRIN-1 specifically expressed at the surface of cancer cells, and to provide an application as a cancer treatment and/or prevention agent. The present invention identifies a cancer antigen protein specifically expressed at the surface of cancer cells, and provides an application as a cancer treatment and/or prevention agent for an antibody having the protein as a target, specifically providing a drug composition that is for cancer treatment and/or prevention and that is characterized by: containing a heavy-chain variable region containing the amino acid sequences indicated in sequence numbers 5, 6, and 7 and a light-chain variable region containing the amino acid sequences indicated in sequence numbers 9, 10, and 11； and containing an antibody having immunological reactivity with the CAPRIN-1 protein or a fragment of the antibody as an active ingredient.
