This invention provides: a method for detecting a cancer, comprising measuring the expression of a polypeptide having binding reactivity through antigen-antibody reaction with an antibody against CAPRIN-1 having an amino acid sequence shown in any even-numbered SEQ ID NO shown in SEQ ID NOs: 2 to 30 in the Sequence Listing in a biological sample; a method for detecting a cancer which involves determining the presence and the amount of CAPRIN-1 in a sample of a cancer patient in order to determine the administration of a therapeutic drug targeting CAPRIN-1 to the cancer patient; and a drug and a kit for the diagnosis of a cancer, comprising an anti-CAPRIN-1 antibody.