The term Shinno-ningoku means provinces to which imperial princes were appointed as kokushi (provincial governor) as well as such systems.
This system was established on October 14, 826 based on sojo (report to the Emperor) made by KIYOHARA no Natsuno ("Ruiju Sandaikyaku": Shinno-ningoku Daijokanpu (the official document issued by Daijokan, Grand Council of the state)).
The Emperor Kanmu had many princes/princesses and the following emperors, the Emperor Heijo and the Emperor Saga, also had many princes/princesses. As a result, the government faced, in 826, a financial problem in supporting many Imperial Prince's families as well as the shortage of posts to be allocated to Imperial Princes. KIYOHARA no Natsuno pointed out, in addition to the above problems, that the practice of appointing Imperial Princes to hasshokyo (the ministers of eight ministries) was problematic. In order to solve these problems, he proposed the Emperor establish the Shinno-ningoku system. Although this system initially started as one which was valid only during the era of the Emperor Junna, it continued to exist and took root throughout the Heian period.
Provinces which were designated as Shinno-ningoku were Hitachi Province, Kazusa Province and Kozuke Province. All of them were major provinces. In the case of these three provinces, Imperial Princes were always appointed to their kokushu, the head of provincial governors.
Imperial Princes who were appointed to the kokushu of Shino-ningoku were called 'Taishu.'
The court rank of Imperial Prince Taishu was necessarily higher than that of other kokushu. While the rank of kokushu were normally from Jugoinojo (Junior Fifth Rank, Upper Grade) to Jurokuinoge (Junior Sixth Rank, Lower Grade), that of Imperial Prince Taishu of Shinno-ningoku was Shoshiinoge (Senior Fourth Rank, Lower Grade).
Those who were firstly appointed to the kokushu of the above three provinces in 826 were Imperial Prince Kaya (Taishu of Hitachi Province), Imperial Prince Nakano (Taishu of Kazusa Province), and Imperial Prince Kadoi (Taishu of Kozuke Province) and all of them were Emperor Kanmu's princes.
As the position of Imperial Prince Taishu was yonin (remote appointment) and Imperial Princes were not required to go to the respective provinces, suke, the deputies of Imperial Prince Taishu, were practically the highest position in Shinno-ningoku. When zuryo-kokushi (the head of provincial governors) appeared in the mid-Heian period, suke were appointed to the zuryo of Shinno-ningoku and were treated equally with kokushu of other provinces. Although it is believed that Taishu of Shinno-ningoku received their salary but gave other revenues to Muhon-Shinno (Imperial prince without court rank) (inclusive of Imperial Princesses without official position), the details are not clear.
After the Johei and Tengyo War, TAIRA no Masakado appointed persons to the kokushi of eight provinces in the Kanto region in the capacity of the new emperor, but he appointed 'Hitachi no suke' (assistant governor of Hitachi Province) and 'Kazusa no suke' (assistant governor of Kazusa Province) to the kokushi of Hitachi and Kazusa respectively. Being a rebel, he was not obliged to observe the custom of Shinno-ningoku, but it seems this custom took root as a tradition. In the case of Kozuke Province, however, he appointed 'Kozuke no kami' (governor of Kozuke Province), though its reason is not clear.
In the Kenmu Restoration period led by the Emperor Godaigo, Mutsu Province was also designated as Shinno-ningoku for a while and Imperial Prince Norinaga actually went to Mutsu Province as the Taishu of Mutsu Province.
Shinno-ningoku continued to exist nominally ever since. The reason why Nobunaga ODA called himself 'Kazusa no suke' and Tadateru MATSUNAGA was appointed to 'Kazusa no suke,' or Masazumi HONDA, Yoshinaka KIRA, and Tadamasa OGURI were appointed to 'Kozuke no suke' was because the use of 'Kazusa no kami' and 'Kozuke no kami' was limited to Imperial Princes, though these titles were only nominal.