A flat inner container (3), especially an internal tank for a road vehicle, which is surrounded by an outer container (1) and is used for receiving a cryogenic liquid, particularly a fuel. The inner container (3) comprises a combination of the following features: a longitudinally extending monolithic base (4) with a top wall (5) and a bottom wall (6) which are connected to also longitudinally extending sidewalls (7), and with at least two longitudinally extending, substantially straight webs (9) that connect the bottom wall (6) to the top wall (5) so as to form at least one longitudinally extending chamber (10) which is arranged between the webs, extends along the entire length of the base (4) as well as from the bottom wall (6) to the top wall (5), and has a predetermined width between the webs; and at least two caps (11) which tightly seal the two open ends of the base (4) at the periphery; the top wall and/or the bottom wall is/are provided with an arch relative to a planar reference top wall and/or reference bottom wall, the distance of the arch between the inner contour of the top wall and/or the bottom wall and the planar reference top wall and/or reference bottom wall amounting to less than 30 percent of the width of the chamber in the center between the webs.