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