The third chamber is as big as a six-mat room, and its surroundings and ceiling are all made of hewn-stone-shaped clayey soil which is built up like a bridge with two semicircular arches.
三室目は大きくて六畳敷くらいの大きさがあり、周囲や天井は全て、滑土を切石状に切り出したものをあたかも眼鏡橋状に組み上げている。