A form front positioned between the shield of a tunnel excavator and a tunnel-lining form movable relative to both the shield and the tunnel-lining form. The form front comprises an annular support positioned with clearance space from the shield and the tunnel-lining form, wherein the annular support has a lipped inner seal pressing on the tunnel-lining form and is held thereto by pressure adjacent the inner covering surface of the annular support, and also has an outer seal held under pressure against the shield adjacent the outer covering surface of the annular support. One such form front which fulfills all the desired practical requirements comprises a circular outer seal which has a symmetrical cross section with respect to a central symmetry plane parallel to the plane of the annular support and is positioned in a circular groove provided in the annular support, and at the same time along the bottom of the circular groove under the outer seal a clamp ring divided into a plurality of clamp segments is adjustable radially with the aid of an adjusting mechanism engaged to the ends of the segments so as to tighten or loosen the form front.