In the present paper the author investigates the relationships between the tenno 天皇 (monarch), the Fujiwara regency and the in 院 (abdicated monarch), in an attempt to clarify the structure of the political decision making mechanism during Japan's medieval aristocratic state period. Recent research on medieval aristocratic politics has tended to refute the conceptualization of aristocrat house government (mandokoro 政所) and the office of the abdicated monarch (in-no-cho 院庁) at political focuses, in favor once again of the such councilor bodies as the jin-no-sadame 陣定 and Dajokan 太政官. According to this predominate opinion, the political power of the tenno waned during the eras of the Fujiwara regency and in governance, as the latter two institutions grabbed real power. However, despite this prevailing image of the period, such scholars as Tsuchida Naoshige and Yoshie Akio (in the same vein as Tsuji Zennosuke and Ryu Kiyoshi before them) have argued that the tenno's power was actually expanded within coalitions first with the regency and later with both the regency and the in. It is from this point of view that the author of the present article has made his investigation of the actual workings of state governance during the early medieval period in Japan. His findings are as follows. First, within the transition process from the ancient ritsuryo 律令 state to the aristocratic state (a process that lasted from the nineth to the twelfth centuries), a new system of noble ranks was set up called shinka no reiho 臣下之礼法, and new forms of royal decorum (tenno saho 天皇作法) came into being, which established the status and role of the monarch as customary law. Secondly, the supreme decision making mechanism within the aristocratic state was a collegial system involving the tenno, regent and the in and mediated by the Dajoan clerical affairs minister, the Shikiji Benkan 職事弁官. This decision making triad handled its affairs through the kurododokoro 蔵人所 (palace affairs office) and the Benkan-Kyoku 弁官局 (Dajokan clerical affairs office) and thus controlled the whole bureaucratic chain of command through the mediation of the Shikiji Benkan. This, as a result, neutralized the bolitical power and influence of such aristocratic institutions as the jin-no-sadame and shokei 上卿, the presiding officer over daily government affairs. For this reason the period in question may be said to have been characterized by "Shikiji Benkan politics" under the political coalition of the tenno, regency and the in. Finally, "Shikiji Benkan politics," which was established during the regency of Ichijo Tenno's reign, was later incorporated into the system of governance by the abdicated monarch. This is why such high level diplomatics as mikyosho 御教書 signed by the regent and inzen 院宣 signed by the abdicated monarch (both being promulgated through the Shikiji Benkan) came to function as royal edicts. It was in this way of issuing important government decisions that the system of political rule by the regency and the in through the Shikiji Benkan was formally established. The characteristic feature of this system was flexibility which allowed for authority in governance affairs to shift easily and smoothly between the offices of the tenno, regent, and in in response to the extremely fluid political situation of the time.