Device for sealing a building opening having a flat closing element and a frame firmly attachable to an edge of the building opening, the flat closing element being a door leaf/casement pivotable relative to the frame, wherein the flat closing element and/or the frame firmly attachable to the edge of the building opening has a frame profile (1, 29, 39, 43, 72, 77, 90, 94, 108, 109), which borders, in its longitudinal direction, a hollow profile section (2, 74, 78) that opens out to a folding surface, by three surfaces arranged relative to each other in U-form, wherein the surfaces have two lateral surfaces, wherein, from each of the two lateral surfaces, a profile wall (5, 79) juts out towards the other lateral surface, which narrows down locally the widths of the hollow profile section when seen in cross section, and wherein fitting parts are attached to these jutting profile walls (5, 79) using clamping elements, by sandwiching the jutting profile walls (5, 79) between the respective fitting part and the respective clamping element and by the clamping element and the fitting part spanning the hollow section, characterized in that a connection between the fitting part and the jutting profile walls is made such that the connection blocks, in a positive-locking manner, a relative movement of the profile walls towards and away from each other, wherein, in order to establish the positive-locking blocking, a touching geometry between the jutting profile walls and the fitting part is made such that the fitting part, on both of the jutting profile walls, abuts on either side of a groove which is formed on each of the jutting profile walls and whose opening area lies parallel to the connecting surface between the two jutting profile walls, wherein a space between either sides of the groove, at those areas, which the fitting part touches, narrows with increasing groove depth. the surfaces have two lateral surfaces, wherein, from each of the two lateral surfaces, a profile wall (5, 79) juts out towards the other lateral surface, which narrows down locally the widths of the hollow profile section when seen in cross section, and wherein fitting parts are attached to these jutting profile walls (5, 79) using clamping elements, by sandwiching the jutting profile walls (5, 79) between the respective fitting part and the respective clamping element and by the clamping element and the fitting part spanning the hollow section, characterized in that a connection between the fitting part and the jutting profile walls is made such that the connection blocks, in a positive-locking manner, a relative movement of the profile walls towards and away from each other, wherein, in order to establish the positive-locking blocking, a touching geometry between the jutting profile walls and the fitting part is made such that the fitting part, on both of the jutting profile walls, abuts on either side of a groove which is formed on each of the jutting profile walls and whose opening area lies parallel to the connecting surface between the two jutting profile walls, wherein a space between either sides of the groove, at those areas, which the fitting part touches, narrows with increasing groove depth.