横穴式石室の前庭について : その起源と系譜

群馬県内の終末期古墳では普遍的に存在する，横穴式石室の前面に広がる前庭構造について，従来は，古墳時代後期に群馬県地域で独自に土着した構造であると解釈されていた。しかし，近年の研究では，全国各地に所在することが明らかとなり，その起源については，古墳時代前期に高句麗地域で成立していることが明確に解ってきた。高句麗では，前庭構造が王陵に採用され，渤海に取って変わった８世紀終末まで連綿と構築されている。一方で，北部九州地方に導入された初期の横穴式石室にも前庭が付されたものがあり，前庭を持つ古墳は，百済，新羅，伽耶地域では認められないことから，これら初期横穴式石室の構築は高句麗の影響化に成立していることが明らかになった。横穴式石室の浸透に伴って前庭が日本各地に拡散していくにもかかわらず，これらを頑なに拒み続けているのが畿内中枢部の大和地域である。おそらく，当時の畿内大和勢力は，外交をはじめとして百済との結び付きを重視しており，こうした状況は，敵対する高句麗との間に一線を画していた結果を反映していると推定される。これに対して，九州で受容された前庭は，その「ハ」の字形に開いた形状が横穴式石室の羨道部の形態に影響を及ぼし，変質を遂げた形で日本各地へと拡散していく。また６世紀代になって美濃，上野周辺地域には九州とは別系譜で導入されると見られ，定着して墓制の主流となっている。埴輪祭祀が終焉した７世紀代の上野地域では，3000基以上の古墳に前庭が構築され，墓前祭祀が営まれていたと考えられる。これらに関与した造墓集団は後に，東国経営に関連して，東北地方へと赴き，任地で古墳が消滅するまで同様の墓造りに勤しんだものと思われる。

A frontal platform structure of the tunnel burial chamber was widely constructed during the final Kofun period in Gunma prefecture. The traditional view accounts that this platform structure was autochthonously developed at the end of the Kofun period in the Gunma region. However, recent archaeological surveys reveal that it is distributed over a wide geographic area throughout Japan. Moreover, its origin may go back to the beginning of the Kofun period in the Kōkuri state of the Korean peninsula.The royal graves equipped with the frontal platforms have been built in the Kōkuri state, and their construction continued up to the end of the eighth century in the state of Bokkai. When the tunnel burial chambers were first introduced to northern Kyushu in Japan, some burials were made in this Kōkuri style. This specific burial structure has not been found in the other Korean states, such as the Kudara, the Shiragi nor the Kaya. Therefore, it is safe to say that these early tunnel burial chambers in Japan developed under the influence of the Kōkuri state.Although the tunnel burial chambers with frontal platforms gradually spread into other regions of Japan, they had never come into the Yamato region. This is because the powerful clans in the Yamato region had an alliance with the Kudara state, which was against the Kōkuri state. This political climate inhibited the adoption of the Kōkuri style burials in the Yamato region. In the Kyushu region, on the other hand, the fan-shaped frontal platforms were introduced without resistance. And the tunnel burial chambers with variably modified entrance corridors began to diffuse over the other areas of Japan from Kyushu.The adoption of burial structures from the Korean state has occurred at multiple times through a different route, In the Mino and the Kouzuke regions, the platform structure was first introduced in the sixth century from a route other than the Kyushu, and flourished thereafter. In the Kouzuke region, more than 3,000 burial mounds with frontal platforms were constructed in the seventh century, when the haniwa rituals had already been abandoned. These frontal platforms were possibly made for the internment ritual. As the Yamato state extended its political boundary, the mound builders in the Kouzuke region moved into northern Japan, where they constructed the burials with a similar structure.

source:https://www.rekihaku.ac.jp/outline/publication/ronbun/ronbun4/index.html#no82