Hyūga Province
The district was once written "囎唹郡," and it was part of Ōsumi Province. The district broke off into Higashisoo and Nishisoo Districts in May 1878. When the district government was activated on April 1, 1897, Nishisoo was merged with Aira District and Higashisoo was merged with Minamimorokata District (broke off from Morokata District in the former Hyūga Province) to become Soo District.