A method of forming an overlapping section involves radiating a plasma onto a first overlapping part of a first workpiece, over which oil is spread. Thereafter, silicone rubber, which functions as an adhesive and a sealant material, is applied onto the first overlapping part, and a second overlapping part of a second workpiece is overlapped on the first overlapping part. When the components are left standing still for a predetermined period of time, the overlapping section is formed, in which the first overlapping part and the second overlapping part are adhered to one another.