The invention relates to a protective frame for mounting casings, of the type that is used in construction for the later installation of metal doors in general and fire doors. The invention is essentially characterised in that the pre-formed metal casing is internally equipped with a protective frame which provides same with the necessary stiffness for handling during transport and subsequent mounting upon being fixed to the external or internal wall of a building. Once the casing setting time has elapsed, the protective frame is removed and the casing is prepared for the installation of the door when necessary, thereby preventing possible damage to the paint on the door owing to impacts, scratches or marks. The invention is advantageous in that it provides improved convenience and precision in terms of mounting and only one person is required for same, thereby considerably reducing manpower while eliminating any possible damage to the door leaf.