A method for detecting a cancer(s) based on an expression of prescribed polypeptides is disclosed. These polypeptides were isolated, by the SEREX method using a cDNA library derived from canine testis and serum from a cancer-bearing dog, as a polypeptide which binds to an antibody existing in serum derived from cancer-bearing living body. Because these polypeptides react with antibodies specifically existing in serum of a cancer patient, cancers in a living body can be detected by measuring the antibody in a sample. Cancers in a living body can also be detected by measuring the antigen protein of the antibody per se or mRNA encoding it.