Although it is generally believed that Emperor Uda during the Heian period used this title for the first time, because "Shoku Nihongi" (Chronicles of Japan Continued) says that Emperor Shomu joined the ceremony to consecrate the Great Buddha at Todai-ji Temple under the title of Hoo after his abdication of the throne, its origin is not unified.
平安時代の宇多天皇がこの称号を初めて使ったとされているが、『続日本紀』には聖武天皇が譲位して法皇と称して東大寺の大仏開眼供養に参列していたことから、その出所には若干のぶれが生じる。