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