Located in Otaki Town, in Isumi-gun, Chiba Prefecture, the Sonan Museum was inaugurated on September 10, 1975 with the theme "Boso Castles and Castle Towns". The castle that existed in the area from the 17th century up to the middle of the 19th century was restored and remodeled into a museum. In order to preserve tangible reminders of the old castle, some of the adjoining facilities, such as the well and the gate , have been reconstituted. Later, in 1983, study rooms were built adjacent to the museum and are still being used today. The museum organizes 2 or 3 exhibitions a year, as well as practical courses and lectures aimed at promoting awareness for the preservation of cultural assets.

The Sonan Museum was consolidated into the Natural History Museum and Institute Chiba in 2006. Now this museum is commonly known as Otaki Castle Museum.