From the Wikipedia page  - which gives a history of the archives and lists the various subdivisions.
The State Archives of Belgium is a Federal scientific institution which is responsible for managing the Belgian archival heritage. The institute provides public services and performs research in the domain of archiving and the institutional history of the Belgium. The institute supervises the archives built up by the public authorities, acquires archives of authorities and individuals and conserves the archival heritage of Belgium.
The website, available in several languages including English, is