To prevent a fastener from being dropped even when a mounting hole is round, oblong, or rectangular.

While elastic pieces 13; 14 are inserted into a mounting hole 7 of a support member 5 and a peripheral wall 15 is in contact with a plate surface of the support member 5 to secure a space not restraining the action of an engagement part 18 between the plate surface of the support member 5 and a plate surface of a clip 1, with the closure of a cap 3 in a direction of covering the clip 1 about a hinge 4, a protrusion 17 enters a gap between the elastic pieces 13; 14 from a through-hole 12 to press and widen the elastic pieces 13; 14 so that the first stage engagement is established, in which the elastic pieces 13; 14 are engaged with a peripheral part of the mounting hole 7. Further, with the closure of the cap 3 in the direction of covering the clip 1 about the hinge 4, when a claw 34 is in contact with a peripheral surface of the through-hole 12 to bend the engagement part 18 and the claw 34 goes through the through-hole 12, the bent engagement part 18 is restored, and the second stage engagement is established, in which the claw 34 is engaged with the peripheral part of the through-hole 12.