The Diet enacted a special single-use law Friday that will allow Emperor Akihito, 83, to abdicate due to his advanced age, paving the way for Crown Prince Naruhito, 57, to rise to the Imperial throne.

His abdication, which is expected to take place at the end of 2018, would be Japan’s first in about 200 years, since current law only allows Imperial succession to take place when an emperor dies.

The government will decide on the timing by issuing an ordinance.

The abdication will usher in a change in Japan’s era name, or nengo. Japan concurrently uses both Western and traditional calendar systems. The current nengo is called Heisei 29, which refers to the 29th year of the era of Emperor Akihito.

The special abdication law emerged after Emperor Akihito hinted at his desire to step down in a rare televised video message aired last August that cited concerns his advanced age was interfering with his public duties.

On Friday, the Upper House enacted the law with support from almost all of the major parties.

Prime Minister Shinzo Abe told reporters afterward that the process of legalizing the first abdication in modern Japan reminded him of its significance in relation to the “fundamentals of the nation, its long history and future.”

“A stable succession of the Imperial throne is a momentous issue. The government will advance debate with respect for the resolution,” he said.

The public’s attention is now likely to shift to the event’s timing, and whether Abe’s Cabinet will carry out any other reforms to the Imperial succession system.

Concerns over the sustainability of the male-only succession system have been growing because the Imperial family only has one young heir, Prince Hisahito, age 10.

The other seven unmarried children are all females, and six are in their 20s and 30s. Under the Imperial Law, women are obliged to abandon their Imperial status if they marry a commoner.

“The number of Imperial family members is decreasing because of the marriage of female members and other reasons,” Chief Cabinet Secretary Yoshihide Suga told a special Upper House session on the legislation Wednesday.

“Considering the ages of the Imperial family members, this is an important problem that we cannot shelve,” he said.

Many politicians and intellectuals have called for drafting legislation to allow a female to become a reigning empress and establish a branch in the Imperial system even if she marries a commoner.

But Abe is reportedly reluctant to carry out such a drastic reform because many conservative politicians want to retain the traditional male-only, paternal-line succession system.

Abe “strongly prefers” the current system, a close aide said.

During Wednesday’s session, Suga said Abe’s Cabinet “will firmly maintain” male-only succession.

The Imperial House Law says the Imperial throne shall be succeeded only “by male offspring in the male line belonging to the Imperial lineage.” This means a successor should be a male whose father is from the Imperial family.

For example, even if Princess Aiko, 15, a granddaughter of Emperor Akihito and the only child of Crown Prince Naruhito and Crown Princess Masako, marries a commoner and gives birth to a boy, the child would not be qualified to ascend the throne because his father is from outside the Imperial family.

Liberal intellectuals and lawmakers have called for reform to allow a female from the Imperial family’s maternal line — or a female whose mother, not father, is from the Imperial family — to ascend throne in light of the growing concerns and the gender equality guaranteed by the postwar Constitution.

Experts say that Imperial succession will be extremely unstable in the long term without major reform.

According to a 2005 government panel on the Imperial succession system, about half of Japan’s 125 emperors — though the first several were believed to be mythical — were children born to a concubine or their descendants.

This means the Imperial family, which is believed to be the world’s oldest monarchy, was maintained for hundreds of years thanks to the concubine system, which was effectively abolished by Emperor Hirohito, posthumously known as Emperor Showa (1901-1989).

“It will be extremely difficult to maintain stable Imperial succession” based on the male-only, paternal blood-line system, the 2005 report read.

Meanwhile, some conservative intellectuals and politicians have argued that the male succession system be maintained by reviving the Imperial status of 11 branch families that were deprived of their privileges by reforms after Japan’s defeat in World War II.

During Wednesday’s Upper House session, independent lawmaker and former Kanagawa Gov. Shigefumi Matsuzawa argued that it would be a “very effective” reform to revive the Imperial status of those families.

But the 2005 report argued that it is unclear how many of those descendants would consent.

The panel also pointed out that those families are very distant relatives of the Imperial family, whose shared ancestors date back to about 600 years ago.

“It is considered difficult to win the nation’s acceptance” on reviving the Imperial status of those families, the report said.

Information from Kyodo added.