Jakugen (year of birth unknown - April 18, 1024) was a monk of the Tendai Sect, who lived during the mid-Heian period. His alias was MINAMOTO no Tokinobu. He was the eighth son of MINAMOTO no Masanobu, who had the title of Sadaijin (Minister of Left) and his mother was FUJIWARA no Bokushi (also pronounced as FUJIWARA no Atsuko). His maternal sibling was MINAMOTO no Rinshi (the legal wife of FUJIWARA no Michinaga). His birth year was uncertain but is estimated that he was born in 968 or 969 by comparing the age of his siblings and the incident with FUJIWARA no Tadanobu.
Tokinobu was the jiju (a chamberlain) at that time, but did not attend Kamo no rinjisai (Kamo shrine, special festival) along with his coworker FUJIWARA no Tadanobu and FUJIWARA no Nobutaka, who was the Kurodo (Chamberlain), and received the disapproval of Emperor Kazan, and Nobutaka being the oldest among the three was dismissed from his post, while Tokinobu and Tadanobu were lectured in front of his parents in consideration of their youth according to "Shoyuki" (the diary of FUJIWARA no Sanesuke). FUJIWARA no Tadanobu was 17 years old at that time, and since the son of MINAMOTO no Masanobu was first assigned with court duty when he was 16 years old (excluding MINAMOTO no Sukeyoshi who became the monjosho (student of literary studies in the Imperial University) once entering Daigaku-ryo (Bureau of Education under the ritsuryo system)), it is believed that he was 16 or 17 years old at that time. In addition, according to "Shoyuki," he was allowed to undergo Shoden (visit the Imperial Court) on July 17, 985. He was assigned later as Ukone no shosho (Minor Captain of the Right Division of Inner Palace Guards) ("Sonpi Bunmyaku" (a text compiled in the fourteenth century that records the lineages of the aristocracy)) but underwent shukke (become the Buddhist monk), following the example of his older brother, MINAMOTO no Tokimichi in the same year ("Shui Ojo-den"(Gleaning passing on to the next life (dying)) of MIYOSHI no Tameyasu stated that he underwent shukke when he was 19 years old during the Tengen period (978 - 983), but while the age seems suitable, the time period is wrong). The motive is uncertain, but it is hard to believe that Tokinobu, who was the son of MINAMOTO no Masanobu, who was Sadaijin, and had the political power which could suppress the Sessho (regent) FUJIWARA no Kaneie at that time, and was probably due to a pessimistic view on life according to "Shui Ojo-den" and "Genko shakusho" (History of Buddhism of the Genko era). "Eiga Monogatari" (A Tale of Flowering Fortunes) depicted Masanobu angered over the consecutive shukke of his sons (Akazome Emon, who was the author of "Eiga Monogatari" supposedly had a relation with Jakugen).
He climbed up Mount Hiei after he underwent shukke and studied under Kakunin and received Ryobu Kanjo a ceremony to confer the rank of Ajari (a master in esoteric Buddhism; a high priest)) on July 26, 990 before becoming the elder disciple of Kokei to receive Taizokai (Womb Realm and physical principles) and Vajradhatu kanjo (ceremony to be the successor) in April 1012. He moved to Ohara, Kyoto City to avoid the conflict between Enryaku-ji Temple and Onjo-ji Temple, and reestablished Shorin-in Temple. Jakugen performed various Kugyo (ascetic) following the Jodo-shinko (the Pure Land faith) and Study, devoted himself to his studies of Hokke faith, and bishamonten, Vaisravana (guardian god of Buddhism) is said to have appeared before Jakugen each time to protect him ("Kokon Chomon ju" (A Collection of Tales Heard, Past and Present) and "Genko shakusho" (History of Buddhism of the Genko era)). His brother-in-law, FUJIWARA no Michinaga occasionally visited Jakugen to hear his lecture as well as being respected by Akazome Emon and many, he was called as 'Ohara Konoe no shosho' (Minor Captain of the Palace Guards from Ohara) from the court rank he had before he underwent shukke. He supposedly hit kin, kei, inverted copper or iron bell shaped like a bowl (sounded when reciting sutras) just before his death and Kassatsu, and passed away after Junen joju (ten invocations bringing about completion) ("Shui Ojo-den").