NOVELTY - The structure has a floor panel (2) that is fixed to pair of mainframe (1), and is comprised of floor curved face (2a) and floor horizontal surface (2b). A floor upper element (3) and a floor lower element (4) are respectively extended at back of vehicle along floor curved face from rear end of mainframe, to reinforce upper and lower surface sides of floor curved face. A pair of battery frame (5) is attached with respect to fixing portion (4a) of mainframe and floor lower element. A battery case (6) is supported on pair of battery frame.    USE - Structure for mounting automotive battery unit on chassis of vehicle e.g. hybrid vehicle. Can also be used for mounting automotive battery unit on electric vehicle, fuel cell electric vehicle, and plug-in hybrid vehicle.    ADVANTAGE - The dimension of height direction of floor lower element fixed to lower surface side of floor curved space of floor panel can be reduced by providing the floor upper element in the upper surface of floor curved space, so that the space for positioning battery case in downward direction can be ensured. Hence the dimension of vehicle width direction of battery case can be made larger than the dimension between pair of mainframes. Since the battery frame is attached with respect to fixing portion of mainframe and floor lower element, the load input from mainframe can be distributed to two-directions at the side of floor upper and lower elements and battery frame. Hence the load which input into the battery case supported on battery frame can be decreased.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the structure for mounting automotive battery unit on chassis of vehicle in vehicle length direction. (Drawing includes non-English language text)    Mainframe (1)    Floor panel (2)    Floor curved face (2a)    Floor horizontal surface (2b)    Floor upper element (3)    Floor lower element (4)    Fixing portion of main frame (4a)    Battery frame (5)    Battery case (6)