In a plastic fuel tank comprising a main body having a laminated structure including a layer made of gasoline barrier material and an outer layer made of common plastic material, and a component part adapted to be secured in a hole formed in the tank main body, the component part includes at least a part made of weldable plastic material and welded to a part of the tank main body made of common plastic material, and a part made of gasoline barrier material and extending across said component part so as to separate the interior and exterior of the tank main body from each other. The two parts of the component part may be permanently integral to each other or may be detachably attached to each other. Thus, the welding strength can be ensured while the permeation of HC through the part made of the common plastic material can be avoided.