In a fuel tank, high-rigidity portions are formed to protrude to an inner side from a part of a wall portion inside the fuel tank, the wall portion defining a closed space. A built-in part is fixed to the high-rigidity portions, and is also fixed to the horizontal portion. Therefore, when the fuel tank is deformed, deformation of the high-rigidity portions is restrained, so displacement of the built-in part is restrained. As a result, deformation is restrained in the horizontal portion of the fuel tank fixed to the built-in part that is restrained from being displaced when the fuel tank is deformed.