In an automotive floor panel assembly (1) comprising a floor panel (1M) stamp-formed of sheet metal and including a longitudinal floor tunnel (2) extending longitudinally in a laterally central part of the floor panel and a lateral floor tunnel (3) extending laterally across the longitudinal floor tunnel, a first reinforcement member (7), which may have a stepped cross section, attached to and extending along a front surface of a front wall (1fw) of the lateral floor tunnel so as to define a closed cross section jointly with the front wall of the lateral floor tunnel. Thereby, a closed cross section is defined jointly by the first reinforcement member and the front wall of the lateral floor tunnel immediately under the front end of the front seats (4) so that the floor panel can be favorably reinforced without limiting the leg rooms of the vehicle occupants of the front seats and without interfering with the space for receiving the fuel tank (10).