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 or a fragment thereof as an active ingredient having immunological reactivity with a partial polypeptide of CAPRIN-1, wherein CAPRIN-1 is represented by any of the even-numbered sequences of SEQ ID NOS: 2 to 30, and wherein the partial polypeptide comprises the amino acid sequence represented by SEQ ID NO: 37 or an amino acid sequence having 80% or more sequence identity with the amino acid sequence.