Saku (佐久郡, -gun) was a district located in Shinano Province (now Nagano Prefecture). Due to land reforms, Saku District split into Minamisaku (南佐久郡) and Kitasaku (北佐久郡) Districts on January 14, 1879.
The former Gunga(ancient district office) (郡衙) is estimated to be located at Nagatoro in the city of Saku.[1][2][3][4]