A rear side member mounted to extend from both sides of a latter half portion of a vehicle body toward a rear of the vehicle is provided. In particular, the rear side member includes fuel tank sections formed as a foremost portion that corresponds to a position of the fuel tank, and rear subframe sections that extend toward a rear of the fuel tank sections and that correspond to a position of a rear subframe. Tire well sections extend backward from the rear subframe sections and correspond to a position of a tire well. Further, the thicknesses of members of the fuel tank sections, the rear subframe sections, and the tire well sections are different.