Uchitsuomi (Naishin) was one of Ryoge no kan (official posts outside of Ritsuryo code) which existed from the Asuka period to Heian period under the government based on the Ritsuryo code. Four persons were appointed to this post in history and all of them came from the Fujiwara clan.
Being the chief adviser to the Emperor, Uchitsuomi presided over important state affairs while assisting the Emperor, and it was an important post comparable to ministers. However, this post was not a permanent one and its role also varied according to time and circumstances.
NAKATOMI no Kamatari (Fujiwara no Kamatari) was appointed to this post for the first time in history after the head family of the Soga clan fell in Isshi Coup (the Murder of the Year of Isshi) which occurred in 645. He assisted the Emperor Tenchi as a pivotal figure in politics who led all government officials and implemented the Taika Reforms. At the time when Isshi Coup occurred, the Nakatomi clan, from which Kamatari came, was a prestigious clan which was in charge of jingi (worshipping) at court, but its past records in politics were inferior to those of the Soga clan, the Otomo clan or the Abe clan. Therefore, there was a possibility that appointing Kamatari of the Nakatomi clan to Sadaijin (Minister of the Left) or Udaijin (Minister of the Right), the highest posts of subjects based on the system, might provoke antipathy among powerful families which were traditionally engaged in politics at court. Under this situation, the extraordinary post of 'Uchitsuomi' was created so that Kamatari could take part in the center of government. On the day before his death, Kamatari was granted Taishokkan (Grand Crown) and was appointed to 'Naidaijin' (Minister of the Center).
The second person who was appointed to this post was FUJIWARA no Fusamae, a grandson of Kamatari and the founder of the Northern House of the Fujiwara clan. When the Empress Genmei was gravely ill in 721, he was expected to serve as the conservator of her grandson Imperial Prince Obito (the Emperor Shomu), and was appointed to this post as a person who was in charge of 'keikai-naigai, junchoku-shiko' (conveying and implementing the Imperial order). Under the situation where four Fujiwara families and Imperial families like Nagayao and so on were at odds, it is said that he was appointed with the aim of paving the way for the enthronement of Imperial Prince. Being Sangi (councilor), Fusamae's status was not so high at Daijokan (Grand Council of State). However, he had the authority to execute the proceedings to convey and implement the orders of the Emperor (in this case, inclusive of the retired Emperor and Imperial Prince) ('keikai-naigai, junchoku-shiko') in place of Daijokan and Chumusho (ministry of Central Affairs). In addition, he was in a position to check Daijokan when necessary since he was in charge of jutoneriryo (the office of Imperial guards).
There is no doubt that Kamatari and Fusamae were engaged in national politics as the close aides of Imperial Prince (later, the Emperor), but the relation between Uchitsuomi and the center of government (under the Ritsuryo system, it was Daijokan) is not clear (Although Fusamae was sangi, it was a low-ranking post under the hierarchy of Daijokan). Rather, it is considered that they were in a position as an advisor to the government who had independent authority from that of the Daijokan. It is also controversial whether Fusamae was removed from Uchitsuomi when Imperial Prince ascended the throne on the grounds that the purpose was achieved or whether he continued to be in office until his death.
The third person who was appointed to this post was FUJIWARA no Yoshitsugu of the Ceremonial House of the Fujiwara clan, a great-grandson of Kamatari and a nephew of Fusamae. When Sadaijin FUJIWARA no Nagate died of disease in 771, Yoshitsugu, being the pivotal figure of the Fujiwara clan, was put into a position to support the Emperor Konin. However, Yoshitsugu was deprived of his official post twice up to then due to his involvement in the Rebellion of FUJIWARA no Hirotsugu caused by his elder brother FUJIWARA no Hirotsugu as well as the assassination plot of FUJIWARA no Nakamaro which he attempted together with OTOMO no Yakamochi and so on. As a result, he was promoted slowly and was Chunagon (vice-councilor of state) at the time. Under such situation, the Emperor Konin appointed him to Uchitsuomi, a post whose requirement was Dainagon (chief councilor of state) in principle, treated him equally with ministers (at that time, the Emperor said 'Uchitsuomi's duties are same with those of Dainagon and its treatment follows that of minister,' and a regulation stipulating that Uchitsuomi's court rank and so on was same with Dainagon and its jikifu (vassal household allotted to a courier) was 1000 households, the middle of Dainagon and ministers, was established) and put him into a position of the center of the government by posting him at Daijokan. Further, as Udaijin ONAKATOMI no Kiyomaro, a senior of the same clan, was his superior at that time, it is considered that the aim of Yoshitsugu's appointment was to enable him to preside over the job of Daijokan with Kiyomaro being at the top of Daijokan's hierarchy. In order to reward Yoshitsugu's service, Uchitsuomi was upgraded six years later to Naidaijin modeled after the case of Kamatari, and formally became the ministerial post as a Ryoge no kan.
The last person was FUJIWARA no Uona, a close aide of the Emperor Konin and the fifth son of Fusamae. He was appointed in 778 after Naidaijin Yoshitsugu died in the previous year. Although Uona was Dainagon at the time, it is considered that his political footing was the same as that of Yoshitsugu since ONAKATOMI no Kiyomaro remained in the same position. Although the title of this post was renamed 'Chushin' within one month after Uona's appointment, Uona was appointed to Naidaijin in the following year. Since then, the post of Naidaijin was fixed, and Uchitsuomi naturally disappeared after it turned into Naidaijin, an extraordinary ministerial post of Daijokan (since then, FUJIWARA no Takato and others were appointed to Naidaijin).
Incidentally, there is a theory arguing that Uchitsuomi was created based on the government organization system in Korean peninsula on the ground that there were cases in Korean peninsula where Gaisobun SEN was appointed to Uchitsuomi in Gogureyo and the head of Sahirashiki (a governmental office in Baekje managing clerical works) was called 'Uchitsuomi sahei' in Baekje.