[en] Elementary particle physics explores a fundamental principle of the nature mainly by two means, energy frontier and intensity frontier. On the side of the high energy frontier, experiments using high energy beam gives possibility to produce and examine new particles. On the side of the intensity frontier, experiments using intense beam gives opportunity to precisely measure parameters of known elementary particles and to search for undiscovered rare reactions. Muon physics is playing one of the most important roles in the intensity frontier, and a new upper limits on a branching fraction of μ → eγ decay given by MEG experiment using the most intense continuous muon beam at PSI made the most stringent limit on some theoretical models. J-PARC offers excellent experimental site in muon physics by the most intense pulsed muon beam, and several experiments are planed to be conducted at J-PARC. (author)