A measuring cable travel sensor for detecting the position of a movable body and operable under differential pressure comprises a cable drum for winding on and off a measuring cable, a spring for prestressing the cable drum to wind on the measuring cable and a rotary position sensor for detecting the position of the cable drum. The sensor has a differential pressure chamber which is under the differential pressure and in which a cable eye for connection of the cable to the movable body is arranged, a cable chamber in which the cable drum is arranged and a spring chamber in which the spring is arranged. The differential pressure chamber communicates at least with the cable chamber and the spring chamber.