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April 28, 866 (Jōgan 8, intercalary 3rd month, 10th day)
Ōten Gate, Heian Palace (present-day Kyoto)
約12分鐘閱讀
During the Jōgan era (859–877), the Heian court operated under a regime increasingly dominated by Grand Minister Fujiwara no Yoshifusa, maternal grandfather of the boy emperor Seiwa, who had ascended the throne in 859 at the age of nine. Yoshifusa had inherited the standing of the Fujiwara Northern House established since Emperor Saga's reign, and had secured the succession by marrying his daughter Akirakeiko to Emperor Montoku, whose son became Emperor Seiwa. Yet even as the Northern House's power expanded, other powerful non-Fujiwara families still held key posts at court — notably Minister of the Left Minamoto no Makoto, a son of Emperor Saga who had been made a subject, and Major Counselor Tomo no Yoshio, scion of the ancient Ōtomo clan with a lineage reaching back to the Kanmu court.
Yoshio, a capable administrator of considerable learning, had risen through the court bureaucracy and hoped for promotion to Minister of the Right — a post effectively blocked by Minamoto no Makoto's seniority as Minister of the Left, and complicated further by rivalry with rising Fujiwara councilors. It was within this delicate balance of power that a dramatic event shook the political history of Heian-kyō in 866 (Jōgan 8).
On the night of the tenth day of the intercalary third month of Jōgan 8 (April 28, 866), the Ōten Gate — the main southern gate of the Greater Imperial Palace, used for major ceremonies such as New Year's audiences and enthronements — was set ablaze by unknown arsonists. Its destruction was a grave matter for the court. In the aftermath, Minister of the Right Fujiwara no Yoshimi and Major Counselor Tomo no Yoshio accused Minister of the Left Minamoto no Makoto before the Council of State of having set the fire.
Yoshimi moved to have Makoto arrested by force, but Grand Minister Fujiwara no Yoshifusa intervened upon learning of the situation, arguing for Makoto's innocence and working to calm the affair. Thanks to Yoshifusa's defense, Makoto escaped formal punishment, though the episode badly damaged his political standing and he subsequently withdrew from public life. At this point, the true culprit behind the fire remained unidentified.
The affair took a decisive turn on the third day of the eighth month of that year, when a minor official from Bitchū Province named Ōyake no Takatori came forward to denounce Tomo no Yoshio and his son Tomo no Nakatsune as the true arsonists. Though this denunciation is said to have stemmed from a personal grudge between Takatori and one of Yoshio's retainers, it prompted the court to open an investigation into the father and son.
Under questioning, Yoshio and Nakatsune were gradually cornered and eventually confessed to involvement in the arson. On the twenty-second day of the ninth month of Jōgan 8, the court formally condemned them: Yoshio was exiled to Izu Province and Nakatsune to Oki Province. Eight other kinsmen of the Tomo clan were implicated and exiled as well. Through this affair, the Tomo clan (formerly the Ōtomo, renamed to avoid using the personal name of Emperor Junna) — an ancient and once-illustrious lineage — lost its standing at court entirely and thereafter vanished from the historical stage.
In managing the resolution of the Ōtenmon affair, Fujiwara no Yoshifusa played the central role, demonstrating his political skill and influence both within the court and beyond. In the course of settling the incident, Yoshifusa — acting as guardian to the young Emperor Seiwa, then barely into his teens — is said to have become the first subject outside the imperial family to hold the formal office of regent (sesshō) in Japanese history. This was a landmark development: an office that had, in principle, been reserved for members of the imperial family was now held by a member of an aristocratic lineage, the Fujiwara — establishing the direct starting point for the regency-and-chancellorship (sekkan) politics that would define the Fujiwara clan's dominance throughout the Heian period.
The Ōtenmon Incident silenced Minamoto no Makoto, an influential figure of imperial descent, and eliminated the ancient Tomo clan altogether. As a result, rival power centers within the court retreated significantly, clearing the way for the lineage of Yoshifusa and his adopted heir Mototsune (himself a future regent and chancellor) to monopolize leadership at court. Later historians have cited this sequence of events as a paradigmatic case of the "elimination of rival clans by the Fujiwara," ranking alongside the exile of Sugawara no Michizane (the Shōtai Incident) as a symbol of the Fujiwara clan's relentless consolidation of power during the Heian period.
The course of the Ōtenmon Incident is recorded in official histories such as the Nihon Sandai Jitsuroku, and was later vividly depicted in the picture scroll Ban Dainagon Ekotoba (a National Treasure attributed to Tokiwa Mitsunaga), which illustrates the discovery of the fire through Yoshio's arrest in a sequence of dramatic scenes. Highly valued as an art-historical record of Heian court costume, architecture, and ceremony, the scroll shows how this political affair transcended mere documentary record to become an important subject in the history of Japanese art as well.
The Ōtenmon Incident itself was a courtly political scandal involving no armed conflict, and no record survives of swords being used directly in the affair. Nevertheless, the early Heian period was precisely the era in which the technical transition from the straight-bladed chokutō to curved blades — the direct ancestor of the later Japanese sword — was underway. The ceremonial tachi carried at court and the arms borne by the guard officers who escorted Tomo no Yoshio into exile offer a glimpse of the military-official system and sword culture of the time, making the incident a point of reference in studies of early Heian sword history. Moreover, as Fujiwara power consolidated in the wake of this affair, the court's military strength increasingly came to depend on provincial warrior clans — the ancestors of the later Minamoto and Taira — making the Ōtenmon Incident an important touchstone for understanding the long historical transition in Japan from aristocratic rule to the age of the samurai.