A fitting frame having improved prefabricated rails, the fitting frame being implemented so as providable on an interior wall surface and the like, and includes: an upper frame body and a lower frame body which have facing surfaces respectively facing the upper end portion and the lower end portion of a door frame, and which are extrusion-molded in a longitudinal direction; and rail assemblies which are respectively fastened to the facing surfaces of the upper frame body and the lower frame body, and which guide the door frame so that same is mounted and slides thereon.