A tank joint part has a welding member welded to the outer surface of a fuel tank and a main body formed of a material of low fuel permeability and welded or otherwise joined to the welding member. The welding member is formed of modified polyethylene, which is easily weldable or otherwise joinable to the outer surface of the fuel tank and the main body, and in which very fine particles of a layered mineral are easily dispersible. The mineral is dispersed in layers oriented substantially at right angles to the direction of any possible permeation from the tank joint part. The tank joint part ensures a low fuel permeability and a high bonding strength.