The present invention provides an antibody having as a target a cancer antigen protein specifically expressed at the surface of cancer cells and an application of the antibody as a cancer treatment and/or prevention agent, specifically providing: an antibody or fragment thereof having immunological reactivity with the CAPRIN-1 protein and containing a heavy-chain variable region containing the complementarity determining region of sequence numbers 5, 6, and 7 and a light-chain variable region containing the complementarity determining region of sequence numbers 9, 10, and 11； and a drug composition that is for cancer treatment and/or prevention and that is characterized by containing the antibody or fragment as an active ingredient.
