Its major roles were foreign diplomacy and defense, and for the nine provinces in Saikaido (Chikuzen, Chikugo, Buzen, Bungo, Hizen, Higo, Hyuga, Satsuma, and Osumi Provinces) and three islands (Iki, Tsushima, and Tane (present Osumi Islands, absorbed into Osumi in 824) Provinces), it held jurisdiction over an administrative branch and a judiciary branch including personnel affairs of jo (secretary of provincial offices) and lower ranks as well as auditing shidoshi (envoys who bring four major official documents sent by kokushi (provincial governors)).
外交と防衛を主任務とすると共に、西海道9国（筑前国、筑後国、豊前国、豊後国、肥前国、肥後国、日向国、薩摩国、大隅国）と三島（壱岐国、対馬国、多禰国（現在の大隅諸島。824年に大隅に編入））については、掾（じょう）以下の人事や四度使の監査などの行政・司法を所管した。