A battery unit comprises side frames, cross frames, and plural battery modules having substantially the same size comprising a longitudinal dimension, a lateral dimension, and a height dimension. The battery modules are respectively mounted at the cross frames in the same position and arranged in plural rows adjacently to a vehicle width direction such that a direction of the above-described longitudinal dimension is substantially parallel to a vehicle longitudinal direction, and mounting rigidity of the battery modules stored in a first storage area is set to be different from that of the battery modules stored in second and third storage areas.