In Japan, the system of one era per Emperor was adopted when the era name "Keio" was changed to "Meiji" in 1868 during the Meiji Restoration although, before that, an era name had sometimes been changed during the reign of an Emperor.
日本で一世一元の制が採られるようになったのは、明治維新の際の1868年の慶応から明治への改元からであり、それ以前は天皇の在位中にも改元がなされていた。