The list below contains all people currently eligible to succeed to the throne.
The Imperial House Law of 1889 was the first Japanese law to regulate the imperial succession. Until October 1947, when it was abolished and replaced with the Imperial Household Law, it defined the succession to the throne under the principle of agnatic primogeniture.
In all instances, the succession proceeded from the eldest male heir to the youngest (Ch. I: Article 3). In the majority of cases, the legitimate sons and male heirs of an emperor were favoured over those born to concubines. Illegitimate sons would only be eligible to succeed if no other male heirs existed in the direct line; however, the illegitimate sons of an emperor had precedence over any legitimate brothers of the emperor (Ch. I: Article 4). Those in the line of succession suffering from "incurable diseases of mind or body," or when "any other weighty cause exists," could be passed over with the advice of the Imperial Family Council, headed by the emperor, and after consulting the Privy Council (Ch. I: Article 9).
On 11 February 1907, an amendment was made to the Imperial House Law to reduce the numbers of imperial princes in the shinn?ke and ?ke, the cadet branches of the imperial family, who were fifth- or sixth-generation descendants of an emperor. The amendment provided for princes to leave the imperial family, either by imperial decree or by imperial sanction. They were then granted a family name and assumed the status of nobles with the peerage titles of marquis or count, thereby becoming subjects (Article I). Alternatively, a prince could be formally adopted into a noble family or succeed to the headship of an imperial family line as a noble (Article II). Under the terms of the amendment, those former princes and their descendants who left the imperial family were excluded from the line of succession and made ineligible to return to the imperial family at any future date (Article VI).
As of October 14, 1947, when the Imperial Household Law abolished the shinnoke (Princely Houses of the Blood) and oke (Princely Houses) cadet branches, the immediate line of succession to the Japanese throne was as follows:
Prior to this date, the imperial succession was defined by the Imperial House Law of 1889. As the Taish? Emperor had no brothers, if the main family line had become extinct, the imperial line would have continued through the Fushimi-no-miya shinnoke cadet branch under the terms of the 1889 house law. The Fushimi-no-miya house constitute the nearest direct-male line of imperial descendants; the princes of this branch were descended from Prince Fushimi Kuniie (1802-1872), a 12th-generation descendant of the Northern Court pretender "Emperor" Suk?, who was himself the grandson of the 93rd emperor Go-Fushimi. Prince Fushimi Kuniie had 17 sons, 3 of which were by the prince's wife Princess Takatsukasa Hiroko (including his future heirs, Prince Sadanori and Prince Fushimi Sadanaru) and the rest were all by various concubines, of whom five begat oke that were extant as of 1947. However, the sons bore by Princess Hiroko and their descendants had more prior right to succeed the throne since Hiroko was the only official wife of Prince Kuniie. A 1907 amendment to the Imperial House Law further reduced the number of imperial princes eligible to succeed to the throne. By the amended 1889 house law, the imperial line of succession continued as follows:
The Nashimoto collateral branch became extinct in 1951, followed by the Yamashina and Kan'in branches in 1987 and 1988. The main Fushimi-no-miya line and the Kuni, Kaya, Asaka, Higashikuni, Takeda and Kitashirakawa collateral branches remain extant as of 2015. The present head of the Fushimi-no-miya family and the head of the Kitashirakawa branch lack male heirs to continue their lineages, however.
Debate over the imperial succession was first raised in the late 1920s after the Sh?wa Emperor's accession. For the first eight years of their marriage, the emperor and empress only had girls; as a result, the emperor's younger brother, Prince Chichibu, remained first in line and heir presumptive to the throne until the birth of Crown Prince Akihito in December 1933. As a career military officer and known nationalist with radical leanings, the prince enjoyed close relations with the rightist faction in the military. During the early 1930s, his strong support for the "Imperial Way" faction in the army was an open secret; he cultivated strong friendships with several junior officers who were later instrumental in leading the revolt during the February 26 Incident.
A large number of "Imperial Way" followers in the military were critical of the emperor for his scientific interests, self-effacing demeanour and presumed pacifism, considering him a "mediocre" individual easily manipulated by corrupt advisors. With his political leanings, Prince Chichibu antagonized his elder brother, who strongly reprimanded him on several occasions and arranged for his posting to unimportant positions where he could be more closely watched. Apart from Prince Chichibu, the February 26 rebels relied on the tacit support of Princes Asaka and Higashikuni, both senior army generals and imperial princes who were leaders within the "Imperial Way" faction and had close ties to prominent rightist groups. If the emperor had either died or had been compelled to abdicate, Prince Chichibu would have received strong support from the rightists as the regent for Crown Prince Akihito; however, he was reported to have distanced himself from the "Imperial Way" officers following the suppression of the February 26 revolt. Still, in 1938 Prince Saionji expressed his worry that Prince Chichibu might someday usurp the throne by violent means. By October 1940, however, Prince Chichibu had become seriously ill with pulmonary tuberculosis, and led a retired life from then on. He was quietly passed over in the line of succession in favour of his brother Prince Takamatsu, who began to undertake more official duties. In an emergency, Prince Takamatsu was intended to assume the regency for his nephew the Crown Prince.
In July 1944, though the hopelessness of Japan's war effort became clear after the loss of Saipan, the emperor persisted in defending Prime Minister Tojo and his government and refused to dismiss him. Recognising the emperor's continued obstructiveness would lead to certain defeat, Marquess Kido Koichi, the Lord Privy Seal, quietly consulted with Konoe Fumimaro and the emperor's uncle General Prince Higashikuni Naruhiko about the possibility of forcing the emperor to abdicate in favour of his son the Crown Prince and declaring a regency with Prince Takamatsu as regent. On 8 July, the decision was formally taken, with Prince Takamatsu endorsing it several days later. By this plan, Prince Higashikuni would replace Tojo as prime minister and attempt to negotiate a settlement with the Allies. However, the plan was ultimately dismissed as being too risky. Konoe had informed Kido of rumours that if such a situation were to arise, radicals in the military would stage a coup and take the emperor to Manchuria, still considered a safe location for a government, or replace him on the throne with a more militant imperial prince. In the event, Kido and Konoe used the influence of Prince Takamatsu and his uncles the Princes Asaka and Higashikuni to pressure the emperor to ask for Tojo's resignation; this strategy proved successful, and Tojo resigned his posts on 18 July.
Article 2 of the Constitution of Japan provides that "The Imperial Throne shall be dynastic and succeeded to in accordance with the Imperial Household Law passed by the Diet." The Imperial Household Law of 1947 enacted by the 92nd and last session of the Imperial Diet, retained the exclusion on female dynasts found in the 1889 law. The government of Prime Minister Shigeru Yoshida cobbled together the legislation to bring the Imperial House in compliance with the American-written Constitution of Japan that went into effect in May 1947. In an effort to control the size of the imperial family, the law stipulates that only legitimate male descendants in the male line can be dynasts; that naishinn? (imperial princesses) and jo? (princesses) lose their status as imperial family-members if they marry outside the imperial family; that shinn? (imperial princes), other than the crown prince, ? (princes), unmarried imperial princesses and princesses, and the widows of imperial princes and princes may, upon their own request or in the event of special circumstances, renounce their membership in the imperial family with approval of the Imperial House Council; and that the Emperor and other members of the imperial family may not adopt children.
For an imperial abdication to take place, such as the one that took place in April 2019, it requires special legislation and cannot be explicitly expressed by the monarch himself.
Before September 2006, there was a potential succession crisis since no male child had been born into the imperial family since Prince Akishino in 1965. Following the birth of Princess Aiko, there was significant public debate about amending the Imperial House Law to allow female descendants of an emperor and their descendants to succeed to the throne. In January 2005, Prime Minister Junichiro Koizumi appointed a special panel of judges, university professors, and civil servants to study changes to the Imperial House Law and to make recommendations to the government.
On January 24, 2005, the Japanese government announced that it would consider allowing the Crown Prince and Crown Princess to adopt a male child, in order to avoid a possible succession disputes. Adoption from other male-line branches of the Imperial Line is an age-old imperial Japanese tradition for dynastic purposes, prohibited only in modern times after the adoption in 1947 of the American-written Constitution of Japan. The child would presumably be adopted from one of the former imperial branches which lost imperial status after World War II. However, a government-appointed panel of experts submitted a report on October 25, 2005, recommending that the imperial succession law be amended to permit absolute primogeniture.