To reduce load that can be input to a battery unit, while solving the dimension limitation of the battery unit, concerning a mounting structure of the vehicle battery unit.

A floor panel 2 is fixed to a main frame 1 that extends to a longitudinal direction of a vehicle, a floor bending surface 2a is provided by bending the rear side to the upside and a floor horizontal plane 2b is provided by further extending horizontally from the rear end. Moreover, a floor upper member 3 and a floor lower member 4 are extended from the rear end of the main frame 1 to the backward of the vehicle along the floor bending surface 2a, and a top surface side and a lower surface side of the floor bending surface 2a are reinforced, respectively. Moreover, a first battery frame 5 is mounted to a fixed part 4a of the main frame 1 and the floor lower member 4 and extended to the backward. A battery case 6 is supported on the first battery frame 5.