The aim of this article is to examine the document form known as kei 啓, which was adopted in Japan from China during the 7-8th century, through an analysis of the historical sources in which the term appears and a comparison with its use in China.<br> 　 The author begins by pointing out the use of kei in sources such as the Nihon Shoki 日本書紀 official history and Fudoki 風土記 provincial gazetteers without reference to the emperor, as distinguished from sources such as the glosses to the Nihon Shoki and the receipt for the Tenjukoku embroidered hanging （Tenjukoku Shucho Mei 天寿国繡帳銘）, which confine the term solely to the emperor.<br> Next, the author describes the use of kei in Six Dynasties China as a widely utilized vehicle for petitioning to any higher ranking authority, including the emperor, and as a form for submitting any document to a higher authority other than the emperor during the Tang Period. From these varied usages, he suggests that the Japanese utilization of kei vis-a-vis the emperor was due to the influence of the Six Dynasties usage, while its use by personages other than the emperor was influenced by the Tang Dynasty usage. That is to say, in Japan there occurred a transformation from the Six Dynasties to the Tang Dynasty usage; furthermore, the use of kei throughout 8th century East Asia conformed to the Tang Dynasty usage of submitting documents to any higher authority other than the emperor. For example, the kingdom of Bohai customarily used the kei form in written communications with Japan, leading the latter, which was well aware of the Tang Dynasty usage, to consider Bohai as its tributary state, as indicated in the history of Japan’s diplomatic policy towards Bohai.<br> 　 Next, an examination of 7th century wooden documents （mokkan 木簡） reveals the widespread use of kei in petitioning higher authorities （i.e., as joshin monjo 上申文書）, indicating that the Six Dynasties usage was in vogue and had not only been adopted to fit political conditions at that specific time, but also represented an early innovation in documentation governance. The author concludes that the Six Dynasties usage during the 7th century was changed to the Tang Dynasty usage with the promulgation of the Taiho Ryo 大宝令 codes in 701 CE, as indicated in the item on the form of kei in the Code on Official Protocol （Kushiki-Ryo 公式令） specifying its use for submitting documents to a higher authority other than the emperor.<br> That being said, in a wider social sense the Six Dynasties usage survived, but was never identical to the Tang Dynasty usage in form. Nonetheless, as a result of this institutional transformation in kei usage, the document forms for petitioning the emperor were exclusively designated as hyo 表 and so 奏, leading the author to conclude that the transformation of kei was probably intended to make the unique political status of the emperor clearly apparent within the customary documentary forms for official use.