PURPOSE: To obtain simple joining having excellent shear strength by holding a fluororubber between the superposing sections of a joint material made of the fluororubber and heating and thermally fusing the superposing sections.

CONSTITUTION: The superposing surfaces of joints 4, 4 made of a fluororubber are polished, thus forming roughened surfaces 46, 46. A filmy fluoroplastic 7 is held between the polished joints 4, 4, and the fluoroplastic 7 is heated and melted. The fluoroplastic 7 intrudes to the roughened surfaces 46, 46, thus acquiring joint surfaces having shear strength.