A corrugated plate portion of a core panel is sandwiched between an upper panel and a lower panel, and is formed in a shape in which upwardly-facing concave portions, that open toward a side of the upper panel and extend in a vehicle transverse direction, and downwardly-facing concave portions, that open toward a side of the lower panel and extend in the vehicle transverse direction, are lined-up alternately. A sum of contact surface areas of the upper panel and bottom wall portion sides of the downwardly-facing concave portions at the corrugated plate portion is smaller than a sum of contact surface areas of the lower panel and bottom wall portion sides of the upwardly-facing concave portions at the corrugated plate portion.