To easily confirm a fastening state by sandwiching a seal ring by plural housings via a bolt and a nut, interposing a pair of disc springs having opposed projecting surfaces in layers, and buckling these in a plate shape.
An end part of a connecting tube 4 is pressed in both ends of a seal ring 5, a housing 1a of a coupling main body 1 divided into two parts is fitted to the seal ring 5, a projecting strip 1d is engaged with a recessed groove 4a of the tube 4, a bolt is inserted into a bolt hole 1c of a projecting part 1b, and a nut 3 is threadedly installed in a screw part 2a. At this time, a pair of disc springs 6a are previously fitted and installed in layers in a bolt neck part so that projecting surfaces are opposed, and the screw part 2a side is similarly formed. In temporary fastening, a coloring surface 6b of the disc springs 6a can be seen from outside, but when fastening a bolt and a nut 2, 3 by a tool or the like, the disc springs 6a buckle to become a plate shape, so that the coloring surface 6b is brought into close contact with the disc springs and becomes invisible from outside by regular fastening to thereby easily confirm a fastening state to prevent liquid leakage by forgetfulness of fastening and insufficiency.
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