The present invention provides an antibody targeting a cancer antigen protein that is specifically expressed on the surface of a cancer cell, and the use of the antibody as a therapeutic and/or prophylactic agent for cancer. More specifically, the present invention provides: an antibody, or a fragment thereof, that has an immunological reactivity with a CAPRIN-1 partial polypeptide comprising an amino acid sequence represented by SEQ ID NO:5 or an amino acid sequence having a sequence identity of 80% or greater with said amino acid sequence； and a medicinal composition for treating and/or preventing cancer, said medicinal composition being characterized by comprising the aforesaid antibody or a fragment thereof as the active ingredient.
