Imperial Prince Sadazumi (貞純親王)

Imperial Prince Sadazumi (873? - June 15, 916) was an Imperial family member of Japan in the early Heian period. Imperial Prince Sadazumi was the sixth prince of Emperor Seiwa. His mother was the daughter of Prince Munesada. The princes were MINAMOTO no Tsunemoto and MINAMOTO no Tsuneo. Imperial Prince Sadazumi was also called Imperial Prince Momozono.

Imperial Prince Sadazumi held positions such as governor of Kazusa and Hitachi Provinces, which were shinno ninkoku (provinces whose gubernatorial posts were reserved as sinecures for imperial princes), Nakatsukasa-kyo (Minister of the Ministry of Central Affairs), Hyobukyo (Minister of Hyobusho Ministry of Military), and was awarded the rank of Shoshiinoge (Senior Fourth Rank, Lower Grade). With the demotion from nobility to subject of princes Tsunemoto and Tsuneo when they were both given the family name Minamoto, he became one of the forefathers of Seiwa-Genji. However, there is another theory which claims that Seiwa-Genji, which was traditionally believed to be the line of Imperial Prince Sadazumi, was the blood line of Emperor Yozei (Imperial Prince Sadazumi's older brother). It is also said that he was born on March 10, 870.

Seiwa-Genii or Yozei-Genji

There are several hypotheses of the origin of Seiwa-Genji; one of them is the Yozei-Genji Theory. According to this theory, Prince Tsunemoto, who was supposedly the forefather of Seiwa-Genji, may have been the prince of Imperial Prince Motohira, the prince of Emperor Yozei.
The historian Hisashi HOSHINO in the Meiji period stated this theory, which was supported by descriptions between 1897 and 1906 in the family document of the Tanaka family, who were Shinto priests of Iwashimizu Hachiman-gu Shrine, that the Imperial instruction of 1046, which was dedicated to the Mausoleum of Emperor Ojin by MINAMOTO no Yorinobu, explicitly described that: 'the ancestors are Shinbochi, Tsunemoto, Imperial Prince Motohira, Emperor Yozei, and Emperor Seiwa, from newest to oldest.'
However, this is an uncertain theory because the document is in manuscript with a proviso on the back of the Imperial instruction stating that the contents were proofread. Some say that the contents of the Imperial instruction merely refers to the order of accession of Ishikawasho, Kawachi Province, and since Okagami (The Great Mirror) written in the early twelfth century, describes that Buke-Genji (Minamoto clan as samurai families) is the descendant of Emperor Seiwa, many scholars believe that the Seiwa-Genji theory is true.