Abstract: This preparation process for an observation and treatment endoscope for introduction into the human body through an inner duct of a synthetic material cylindrical tube (2) serving as a catheter extending between an external control and observation head (1) and an internal intervention end (3) and having a certain number of cylindrical longitudinal hollow ducts (17, 22, 23) for receiving the illuminating means (21), the observation means (14) and the treatment means (7) is characterized in that it comprises moulding the tube (2) by placing synthetic material in the pasty state around longitudinal mandrels, whose positions and diameters correspond to the future hollow ducts of the tube and ultrafine optical illuminating fibres, solidifying the synthetic material and then extracting the different mandrels.