In welding a component part (20) to a tank main body (10) of a plastic fuel tank of a laminated structure, an annular upright wall (15) is formed in a tank main body wall so as to define an opening therein. An end surface of a weldable layer of the component part is welded to an outer surface of a weldable layer of the tank main body surrounding the annular upright wall to form a first welded part (31) and to an end surface of the weldable layer of the tank main body exposed at the top surface of the annular upright wall to form a second welded part (32) while the HC barrier layers (12, 24) are kept substantially aligned with each other at the interface between the two parts. Thereby, the required HC permeation blocking property and an adequate welding strength can be achieved at the same time.