PURPOSE: To prevent the damage of a connecting joint by using a supporting fixture to form the inserting margin of an inserting pipe and to release the support against an excess load as a buffer for permitting the inserting of an inserting pipe.

CONSTITUTION: A concave part 11 is provided on the contacting face of an elastic fitting ring 10 with an inserting pipe, and connection is given in a contacting face 12 above this concave part 11. When the elastic fitting ring 10 is brought in contact with a catching part 7 as keeping a normal shape, a gap is formed between the top face 6' of the inserting pipe 6 and the inside 7' of the catching part. When an excessive load is given, a stopper part 9 works to release its supporting as a buffer to permit the inserting of the inserting pipe 6, the concave part 11 on the elastic fitting ring 10 of the stopper part 9 is expanded to the outside to form a narrow part 11', and deformed to shape an expanded part 13 for shortening the inserting margin to absorb excessive loads as a buffer.