Abstract: A retaining piece 6 is urged against a pipe A by a push bolt 5, which has an inclined engagement surface 20 in forced engagement with a complementarily inclined engagement surface 16 of a push bolt insertion hole 14. When the pipe A experiences a force tending to dislocate it, the piece 6 is turned such that a ridge-like edge 25 formed at the bottom 23 of the piece 6 wedges into the pipe A, thus preventing the dislocation of the pipe A. The bottom 23 of the piece 6 and also ridge-like edges 24 and 25 formed at the bottom have curved shapes conforming to the outer periphery of the pipe A, so that the edges 24 and 25 can be readily in close contact with the pipe outer periphery. The piece 6 is retained in a ring body 4 by a retainer 7.