According to the present invention, a cancer antigen protein to be specifically expressed on the surfaces of cancer cells is identified and thus the use of an antibody targeting the cancer antigen protein as an agent for treating and/or preventing a cancer is provided. Specifically, the present invention provides a pharmaceutical composition for treating and/or preventing a cancer, which comprises an antibody comprising a heavy chain variable region that comprises SEQ ID NOS: 39, 40, and 41 and a light chain variable region that comprises SEQ ID NOS: 43, 44, and 45 or a fragment thereof as an active ingredient and having immunological reactivity with a CAPRIN-1 protein.