There are provided a pair of side sills, a floor panel, a pair of floor frames, a battery unit provided between the pair of floor frames, a first cross member interconnecting the pair of side sills, and a seat bracket. The seat bracket comprises a protrusion portion which partially covers an upper wall portion of the first side portion and is upwardly spaced apart from the upper wall portion of the first cross member, and the bead portions and the opening portion (low-rigidity portion) are formed at a part of the upper wall portion of the first cross member which is located at a position corresponding to the protrusion portion.