Sankaiki is the name given to the diary of Tadachika NAKAYAMA. Tadachika NAKAYAMA was a court noble who lived from the late Heian period to the early Kamakura period and served as Naidaijin (Minister of the Palace); the name given to his diary (which is written as '山塊' in kanji) is derived from a combination of the last kanji of Tadachika's yago (family name), which is written as '中山' in kanji, and the first kanji of the Chinese name given to the office of minister ('塊門', which is pronounced "Kaimon"). According to an existent document, it was written over the course of about 40 years, from 1151 to 1194. It contains a large number of detailed entries, but quite a few parts have been lost.
Tadachika was given important posts by Goshirakawa-in (the Retired Emperor Goshirakawa) and MINAMOTO no Yoritomo, and the diary's existent period stretches from the Taira clan's rise, through its golden days, to its fall. It has many unique articles on the situation in Togoku (the eastern part of Japan) from the time of the Taira clan's government to the Jisho-Juei War, and is treated as an important historical resource. Although it lacks an article on the Hogen War and Heiji War, the articles on the birth of Emperor Antoku in 1178, his enthronement in 1180, and the enthronement and Daijoe (banquet on the occasion of the first ceremonial offering of rice by the newly-enthroned emperor) of Emperor Gotoba in 1184 are precise, which shows that Tadachika was familiar with chogi (ceremony at the Imperial Court) and the political situation.
Its descriptions of TAIRA no Shigemori's entry to the priesthood, the repair of Owada no tomari (Owada port) by TAIRA no Kiyomori, the uprising of Prince Mochihito, the Battle of Fujigawa and so on differ from those in other war chronicles such as Heike Monogatari (The Tale of the Heike) and Genpei Seisui ki (The Rise and Fall of the Minamoto and the Taira clans), and it displays differences in historical evidence.