A brim-like flexing portion flexes via an abutment to an outer peripheral surface around an installation hole of an installed member, and it sandwiches an outer peripheral portion around the installation hole together with the elastic pawl portion. In determining a set dimension of the brim-like flexing portion, a diameter dimension of a flexing base portion is set in a value between an outer diameter dimension of a reduced-diameter portion and a hole diameter dimension of the installation hole.