Abstract: A joint for coupling tubes forming a large diameter portion near one end of a fluorocarbon resin tube and forming a fastened portion by clamping at a crimp portion of the large diameter portion with a cap nut includes of a main body of rigid resin joint having a cylindrical portion formed a ring-shaped groove and cut a bevel at its inner wall end and a hard cap nut having a inner shoulder formed an acute-angle hook. The major portion of the large diameter portion of the fluorocarbon resin tube is inserted into the ring-shaped groove. A tube clamp portion as a hermetic portion is formed to press hard a tube crimp portion against the inner wall end of the cylindrical portion with the inner shoulder of the hard cap nut by tightening the hard cap nut on an external thread of the outer wall of the cylindrical portion.