An antibody against a peptide having an amino acid sequence AVQGPEETVT QDC (expressed in single letter amino acid code) as represented by SEQ ID: NO. 1 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.
