Cheonji seosangji (天地瑞祥志, Records of Auspicious Omens of Heaven and Earth) is a book of astronomy and Confucian portentology, completed in 666. The author, Sal Sujin (薩守眞, Sa Shouzhen in Chinese) has been considered as an astronomer of Tang, China, but recent studies suggest that he might be the same person as Seol Sujin (薛秀眞), a military tactician of Silla, Korea. This paper approves it by taking a linguistic approach to the text and tracing the circulation of the book in Korea and Japan. Some Chinese characters in the text, such as ``jung`` (中) and ``ji`` (之), turn out to be typical examples of the Idu (吏讀) script, the archaic Korean writing system using Chinese characters, and it clearly indicates that the author is from ancient Korea. The first official use of ancient Chinese constellations by Silla astronomers in 668 was possibly triggered by the introduction of Cheonji seosangji and the quotation from the book in 932 also hints at its Silla origin. Japanese astronomy flourished while Japan stopped dispatching envoys to Tang from 669 to 702, and Cheonji seosangji seems to have been introduced into Japan at that time. It is concluded that Seol Sujin completed the book in Tang in January, 666 and returned to Silla in April, 666.