Provided is a fuel tank having a built-in component placeable with high accuracy. The fuel tank has a built-in component, and the built-in component includes: a carrier as a rigid body having fittings; and pillars having fitted portions to be respectively fitted into the fittings of the carrier; wherein the fittings include at least one slidable fitting having one of the fitted portions of the pillars slidably fitted thereinto due to post-molding shrinkage of the fuel tank, and a fixed fitting having another one of the fitted portions of the pillars fitted thereinto so as not to be slidable, wherein a slidable direction of the at least one slidable fitting is directed toward the fixed fitting.