Miyanosu Kofun is a tumulus mound built at the base of a sandbank projecting into Tokuyama Bay fronting the Setonai Sea. It is said to be a round burial mound, however its scale is yet unknown. Relics were once excavated in 1803 (the 3rd year of the Kyowa era) then reburied and re-excavated again in 1892 (the 25th year of the Meiji era). The main part is considered to be a pit-type stone chamber. Four mirrors currently remain, although single-edged iron swords and Hajiki pottery are said to have been excavated as well. All the mirrors are cast in bronze and three were made in China and one was made in Japan. The Chinese mirrors are Sankakuen-Banryukyo (a triangular-rimmed frame mirror with dragon pattern), Sankakuen-Nishin-Nijukyo (a triangular-rimmed frame mirror with two pairs of deities and beasts) and Sankakuen-Doukoushiki-Shinjukyo (a triangular-rimmed frame mirror with deity and beast pictures facing one way). While they have cracks and defects, every motif is exquisitely detailed in fine casting work. The Japanese mirror on the other hand is a small Naikoukamon-kyo (concatenated arcs mirror) with a marked degree of rusting. While it is regrettable that this finding has not been backed up by academic research, it remains a valuable resource of information on the early Kofun era in the Chugoku region. It is believed that the deceased who managed to possess three valuable Chinese mirrors was part of a powerful family involved in maritime activities in the Setonai Sea area with a close connection to local affluentials.