A floor panel including a protrusion portion which extends in a transverse direction of a vehicle and crosses a floor tunnel. The protrusion portion ensures a rigidity of the floor panel in the transverse direction of the vehicle. Therefore, it is possible to make a floor panel thinner while maintaining its rigidity. In particular, when a cross portion of a floor tunnel and a protrusion portion is protruded more than any other portion of the floor panel, the rigidity of the floor panel is increased. Further, when the protrusion portion is provided under a front seat and a slope face connects the top of the protrusion portion to floor face, a space under the rear side of the protrusion portion is available for disposing a fuel tank. Therefore, space efficiency is improved. Further, the slope face is available for a rear-seat footrest.