（ABSTRACT）

The CP symmetry is preserved in QCD fairly well; The coefficient of the topological term, which leads to the violation of the CP symmetry, should be smaller than 10^{-10}. The smallness is naturally explained in the Peccei--Quinn model; a Nambu--Goldstone boson is introduced, the QCD instanton generates a potential to the boson, and now the parameter is nothing but the vacuum expectation value \simeq 0 of the potential the boson. The "decay constant" of the Nambu--Goldstone boson should be 10^{10}--10^{12} GeV owing to astronomical and cosmological constraints. Now, there are two questions. First, why the global Peccei--Quinn U(1) symmetry is preserved so accurately? Second, what is the origin of the energy scale of the decay constant? This seminar presents a model that gives an answer to these questions. Supersymmetric extension and its possible implication are also discussed.

Reference: hep-ph/0202171 and an article in preparation