A joint part including a welding portion with a rim of an opening of a fuel resin tank as well as a high barrier property on its interface. The joint part has a multi-layer structure including a barrier layer and at least one of an inner layer and an outer layer coaxially laminated on an inner periphery and/or an outer periphery of the barrier layer, respectively, wherein the barrier layer is formed by a compound material composed of one or more of fuel permeation-resistant resin, such as ethylene vinyl alcohol copolymer, polyamide polyester or liquid crystal polymer, and polyethylene, while the inner layer and the outer layer are composed of high-density polyethylene or modified high-density polyethylene.