By suppressing transcription activation by NF.kappa.B, the present invention was demonstrated to suppress alveolar bone resorption in periodontal disease models, and promote restoration of alveolar bone in periodontal disease-caused bone defect models. Therefore, the present invention provides agents for treating, preventing, and improving periodontal diseases and alveolar bone defects caused by surgical operations, said agents comprising as an active ingredient, a NF.kappa.B decoy or such that suppresses the transcription activity of NF.kappa.B.