An antibody against a peptide having an amino acid sequence AVQGPEETVT QDC (expressed in single letter amino acid code) corresponding to the 134-to 146-positions in human BAFF (B cell activating factor belonging to the TNF family) protein which is preferably a monoclonal antibody； a method of producing the above antibody； a medicinal composition containing the antibody； utilization of the antibody； and a method of screening an inhibitory effect or an activating effect on BAFF with the use of the antibody.
