When an operation wall 7 of an inner member 2 is inserted into an insertion portion 16 of an outer member 3 and an engagement piece 17 does not receive a cam operation due to a slanted cam portion 9, contact pieces 32 of the outer member 3 do not correspond to a positioning hole 33 of the inner member 2 and are located in a position so as not to be inserted into the positioning hole 33. When the outer member 3 is inserted into an attachment hole 22 of an attachment receiving member 21 with maintaining a state where the engagement piece 17 does not receive the cam operation due to the slanted cam portion 9, the contact pieces 32 are elastically deformed inwardly and located in a position so as to be inserted into the positioning hole 33.
