文久二年（1862）閏八月，幕府為因應惡化的京都治安以及幕權低落的窘境，於原本負責京都事務的京都所司代之上，新設京都守護職一職，任命親藩中俸祿二十五萬石的會津藩藩主松平容保（1835～1893）担任。松平容保於該年底進入京都，他不僅必須維持當地的治安，還扮演著擔任幕府與朝廷間的中間人，促進雙方友好的角色。 文久三年（1863）五月十日，主張立刻攘夷的長州藩砲擊經過馬關海峽的外國船隻，開始攘夷行動，而與外國陷入交戰狀態，但是幕府與其他諸藩卻坐視不顧，長州藩為擺脫孤立的窘境，轉而致力實現天皇親征，欲藉此促進全國各地起而響應。八月十三日，朝廷發布了天皇移駕大和（今奈良），準備親征的敕命，然而，以孝明天皇（1831～1866）為首，前關白近衛忠熙、右大臣二條齊敬等高層公卿皆主張將攘夷事務委任幕府，不願親征，並想除去與長州藩聯手，提倡親征的三條實美（1837～1891）等少壯貴族。另一方面，天皇親征的敕命會毀壞幕府受朝廷委任掌理國政的正當性，身為幕府在京都的代表，京都守護職當然不能坐視，故而決定一併剷除握有朝權的三條實美等人及敗壞治安的浪士。結果，高層公卿與會津藩聯手發動八月十八日的政變，三條實美等人則與敗走的長州兵一同返回長州。 本稿主要分析政變的實行者於政變前所做的準備，以及其於政變當天的動向。由於政變計畫案僅存與會津結盟的薩摩藩藩士的獻計文書，故先行研究皆以薩摩藩為主謀論之，認為會津藩只是兵力遭到利用的一方。然而，本論根據文獻發現，薩摩藩原本計畫八月十六日起事，卻因天皇反對而失敗。失望的薩摩藩士於是藉酒消愁，直到翌日天皇又再傳手諭，指將藉會津藩的兵力起事，才又重新振作，投入十八日的政變準備工作。由於薩摩藩的計畫於八月十六日以失敗告終，該藩在十八日政變時僅被分派守衛皇宮出入口之任務，失去掌握主導權的機會。八月十八日政變之後，孝明天皇等人取回了朝中的發言權，會津藩所代表的幕府勢力則掌握了中央政局的主導權。對此結果感到不滿的薩摩藩，因而與長州藩聯手，走上倒幕一途。如此看來，八月十八日政變的成功反而促成了薩長同盟的成立，亦成為明治維新的遠因。 / Kyoto Shugoshoku(京都守護職; the Military Commissioner of Kyoto) was created as the higher commander of Kyoto Shoshidai(京都所司代; the ambassador of Bakuhu in Kyoto), for keeping the peace in the city of Kyoto and recovering the authority of Bakuhu(幕府; shogunate). Matudaira Katamori(松平容保), the lord of the Aizu clan(会津藩), was installed as Kyoto Shugoshoku on September 24 1862, and deployed massive amounts of troops to Kyoto by the end of the year. He was not only responsible for policing the city of Kyoto, but was also expected to unify Chotei(朝廷; the imperal court) with Bakuhu. From June 25, 1863, the Choshu clan(長州藩), which insisted on expelling aliens immediately, shelled foreign ships that crossed the Kanmon Cclannel(関門海峡) and was at war with western countries. However, Bakuhu and the other clans didn’t want to open hostilities, the Choshu clan therefore became isolated. In order to cclange the miserable condition, the Choshu clan proposed that the emperor should defeat the enemy by himself, and forced Emperor Komei(孝明天皇) to announce it on August 13. Because of this, Emperor Komei and the other nobles who opposed this announcement, for example, Udaijin(右大臣; the second prime minister of Chotei) Nijyo Nariyuki（二条斉敬） and Zen-Kanpaku(前関白; previous superior regent) Konoe Tadahiro（近衛忠熙）, decided to exclude the Choshu clan and young nobels like Sanjyo Sanetomi(三条実美) who took its side. On the other side, as the representative of Bakuhu in Kyoto, Matudaira Katamori had to prevent the announcement becoming true. Because he was afraid that Bakuhu shell lose the the legitimacy to reign over Japan from Chotei. That is the reason why he decided to exclude the Choshu clan and Sanjyo Sanetomi’s party. He leagued together with emperor Komei, Nijyo Nariyuki and Konoe Tadahiro, and mounted a coup on August 18. Finally, Sanjyo Sanetomi’s party ran away with the troops of the Choshu clan to Choshu domain. This study attempts to explore the position of Kyoto shugoshoku in the coup by analysis of the preparation and the movement of the administers of the coup. Because the schedule of the coup in existence was proposed by the samurais of the Satsuma clan(薩摩藩), who was associated with the Aizu clan, in prior studies, the coup was led by the Satsuma clan, and successed by using the troops of the Aizu clan. But according to the literature, the plan made by the samurais of the Satsuma clan was intending to mount a coup on August 16 and was repealed by Emperor Komei on that day. Those samurais of the Satsuma clan were frustrated and so they drunk together until the next morning. On August 18, they just ordered to guard the gate of the palace, and to forbid Sanjyo Sanetomi’s party get into the palace and let the other nobles get through the gate with using the black list of nobles which delivered by the Aizu clan. In the mean time, the Aizu clan, which recalled the troops going back to the domain and so doubled the force before August 17, was the commander for the all gates control of the palace. Moreover, Matsudaira Katamori attended the royal court for the exclusion of the Choshu clan and Sanjyo Sanetomi’s party. By these evidences I should make the assertion that the Satsuma-lead coup was ended up on August 16, and the Aizu clan was the one which having great effect in the coup on August 18. As the result, the Aizu clan had taken the leadership in the political scene for Bakuhu, so as Emporer Komei and the others retrieved their voice in Chotei.