PURPOSE: To prevent a cap from dropping out and reversing and also provide sure sealing ability by forming an insertion section to be inserted into the open pipe line of a flared coupler and a seal flange which sticks to the flared coupler and a flare nut.

CONSTITUTION: This flared coupler 4 has an open pipe line 10 to be connected coaxially with a pipe body and a thread groove 4a on the outer periphery of a connecting section. A flare nut has a thread groove 2a corresponding to the thread groove 4a of the flared coupler 4 in the inner peripheral surface. A cap 7 clogs the open pipe line 10. In this case, an insertion section 11, which is inserted in the open pipe line 10 and whose outer diameter is smaller than the inner diameter of the opened pipe line 10, and a seal flange 14 sticking to the connection section 13 of the flared coupler 4 and the abutting section 12 of the flare nut 2 are formed on the cap 7. In this constitution, the cap 7 is prevented from dropping out and also from reversing by being attached beforehand to the open pipe line 10, securing sealing ability.