The present invention provides: an antibody that has specificity for MUC 4 having a sugar chain structure that is expressed at a high level in cancer cells； a glycopeptide that constitutes an antigen that is suitable for producing this antibody； and a new means and method for the diagnosis, prevention, and/or treatment of cancer by means of the aforementioned antibody. The present invention relates to a monoclonal antibody against a glycopeptide. The glycopeptide comprises a human MUC 4 tandem unit peptide having an amino acid sequence represented by SEQ ID NO: 1, and an O-linked sugar chain. The O-linked sugar chain is N-acetylgalactosamine (GalNAc) and binds to threonine, which is the 8th amino acid of the tandem unit peptide. The present invention further includes: a glycopeptide for use in the production of a monoclonal antibody against human MUC 4； a method for detecting MUC 4 in a human body fluid sample； a kit that includes the abovementioned monoclonal antibody； and a pharmaceutical composition for the prevention and/or treatment of malignant tumors that contains the abovementioned monoclonal antibody as an active component.
