PURPOSE: To smoothly and reliably discharge plating liquid entering a gap between a pipe and a cap nut to the outside and to prevent corrosion of the pipe and the cap nut by forming the hole part in the insertion part ranging from the flange part of a cap nut to a bottom.

CONSTITUTION: A cap nut (a) has an insertion part 1 formed according to the diameter of a pipe to be connected. A flange part 3 is formed at the upper part of the insertion part 1. Meanwhile, a screw part 4 is protruded downward from the bottom part of the insertion part 1 and a female screw 6 is formed at the inner side thereof. A hole part 7 is formed in a range of from a position right below of the flange part 3 of the insertion part 1 to a bottom 5. Further, a protrusion 8 is protruded from the lower part of the screw part 4. Since the hole part 7 is formed in a manner described above, even when plating liquid enters between a pipe and a cap nut (a), the plating liquid is smoothly and reliably discharged to the outside and the pipe and the cap nut (a) are prevented from corrosion.