Chikushikoku (竹斯国) is the name of a country or land that appears in the Book of Sui Dynasty and History of the Northern Dynasties in the history of China as being in the Japan region.
[1] The country of Chikushikoku appears in "Book of Sui" volume 81, biography 46 "Eastern Barbarians, the country of Buyeo" and "Northern History" volume 94, biography 82.
It is mentioned immediately after the passage "The Emperor of the Land where the Sun Rises" in the letter sent by the Buyeo (Wa) envoy, Tarishihiko [ja], to Emperor Yang of Sui.
The envoys went from Baekje to Takeshima (not an island that has been disputed by Japan and Korea since the late 20th century), Tsushima to Iki, and then arrived at "Takeshikoku.
The reason why "Sui Dynasty" refers to "Hye-qing" and "Northern History" refers to "Hye-se-qing" is that "Sui Dynasty" was Naming taboo.