Abstract: An apparatus for measuring the level of a medium in a container has a plurality of radioactive emitters which are mutually movable in order to be arranged at a different height level in each case in the container, a protective tube reaching into the container with an extent at least partially in the vertical direction, and an elongate holding device for the emitters, which is introducible into the protective tube and to which the emitters are fastened. A storage device for the holding device together with the emitters, a connection from the storage device to the protective tube, and a conveying device for conveying the holding device into the protective tube and out of the latter are also provided, wherein the holding device is configured to be length-invariant.