A distance measuring device for measuring the fill level in industrial tanks, which includes electronic transmitting- and receiving elements for short wave electromagnetic waves, such as microwaves, and a cylindrical waveguide which extends through a separating wall and inside of which is a waveguide window separating the two spaces and made of a material, such as quartz glass, which is transmissive with respect to electromagnetic waves. The waveguide and the waveguide window (3) have conically-shaped sections (5) to provide axial support. To protect the waveguide window against destruction due to thermal expansions and pressure fluctuations, the conical section (5) of the waveguide window (3) is in a correspondingly formed conical socket (8). The socket is mounted with play in a cylindrical bore (9) of the waveguide window (3) and is supported by a shoulder (10) of the waveguide in the axial direction. For support in both longitudinal directions, two waveguide windows (3), each with one socket (8), are arranged in a mirror image configuration, and the waveguide is divided into two detachably joined sections so that, when open, the sockets (8) and the waveguide windows (3) can be placed into the cylindrical bores (9) of the sections.