The imperial decree of the practice of assigning one era name to the reign of each Emperor (一世一元の詔)

The imperial decree of the practice of assigning one era name to the reign of each Emperor is an imperial decree established on October 23, 1868 (the Gregorian calendar) at the same time of changing Keio year 4 to Meiji gannen (Year of 1868) and it is a system for assigning one era name for each Emperor's reign. It is also referred to as the imperial decree of changing the name of the era to Meiji.

Summary
The era name is changed following the accession (to the throne) by a new Emperor. The era name changed to Taisho in July 1912, following the accession (to the throne) by the Emperor Taisho and it changed to Showa in December 1926, following the accession (to the throne) by the Emperor Showa.

Index to Laws in Japan, run by the National Diet Library, mentions that the imperial decree of the practice of assigning one era name to the reign of each Emperor was lost its effectiveness by the Era Name Law which was established in 1979.

Original text
Having advanced the rank, I have been assigned to change the era name with my heart and soul.'
It is truly a typical case of the magnificent imperial reign as well as a standard and unbroken from the past.'
Although I am not perfect, I am honored to take on this assignment with the help of the spirit from the past Emperors and I will serve my role in all affairs of the state's politics myself.'
That is, to change the era name and I would like to start anew with an almighty and magnificent approach.'
The Keio year four is to change and it is now the start of the Meiji era.'
From now on, the old system will be revolutionaries and will practice assigning one era name to the reign of each Emperor permanently.'
Those who are responsible, enforce the new system accordingly.'