Marked loop-formation of the sigmoid colon makes difficult the advancement of a coloscope. After straightening the sigmoid loops, the further advancement of the instrument is facilitated. In some cases, the straightened loops re-form when the instrument is further advanced. With the aid of a steel wire which is introduced into the instrument channel of the fiberscope after the straightening process, renewed loop formation can be advoided. In addition, the steel wire can also be used in the straightening of the transverse colon.