In an automotive floor panel assembly comprising a floor panel (1) stamp-formed of sheet metal and including a recess (21, 22) defining an upper concave surface and a lower convex surface, a compound longitudinal floor frame is formed by a lower reinforcement (24) extending longitudinally across a part of the lower convex surface of the floor panel and an upper reinforcement (25) extending longitudinally across a part of the upper concave surface of the floor panel. The lower and upper reinforcements include a first vertical wall having an upper edge closely following a contour of the lower convex surface a second vertical wall having a lower edge closely following a contour of the supper concave surface, respectively. The compound longitudinal floor frame is thus provided with a substantially constant cross section and extends substantially linearly over the entire length of the floor panel and hereby. Therefore, the height of a lowest part of the floor panel from the ground surface is prevented from being reduced, and the mechanical strength of the floor panel would not be compromised even when the floor pane is provided with a downward projection.