Kosegasawa Cave is located on the left bank of the Muroya River, upstream of the Tokonami River at an altitude of about 200m; the entrance is about 1.5m wide and the cave is about 7m deep. This cave utilizes a crack in a rhyolite cliff.
Kozaburo Nakamura of the Nagaoka Municipal Science Museum excavated the cave between 1958 and 1959. Together with Muroya Cave, many stone tools consisting mainly of earthenware established the beginning of the Jomon period; hunting tools such as projectile points and flint arrowheads were also excavated. Among the stone tools are some related to the Maritime Province, East Siberia, Russia, making them important to clarify the origins of Jomon culture. Since a great number of chips, which are stone scraps from stone tool production, were excavated together with finished goods, the land probably was a location that connected a workshop and distribution.
Thus the excavated materials enabled researching the dawning of the Jomon period on the Japanese Archipelago. They were designated as important cultural property of Japan in 2000 and are housed and exhibited in the Nagaoka Municipal Science Museum that conducted the excavation. In addition to the designated materials in this museum are materials that were donated to old Kamikawa Village; they can be seen in the Aga Town Hometown Museum.