[en] Muon Spin Relaxation Experiments have contributed to understanding the high-temperature superconductivity through (1) Providing the magnetic phase diagram of the copper oxide superconductor, (2) Giving the relation between the condensed electron density n_s and Tc, (3) Clarifying the phase split in the over-dope region, (4) Analyzing the origin of '1/8 abnormality', and (5) Examination of the impurity effect. It is expected that fine variations in the μ SR time spectra can be examined with high intensity muons at J-PARC. Detailed dependency of the spectra on the samples, the temperature and the magnetic fields are also expected to be measured. (K.Y.)