Tsuibushi (envoys to purse and capture), Ryoge no kan (class outside of the Ritsuryo system), was the origin of the Shugo, and was incorporated into the Shogunate's organization when Emperor Goshirakawa approved Kamakura-dono's (lord of Kamakura) establishing Shugo and Jito (manager and lord of manor).
令外官である追捕使が守護の原型であって、後白河天皇が鎌倉殿へ守護・地頭の設置を認めたことによって、幕府の職制に組み込まれていった。