Elastic force originating from an elastic body is applied to gripping portions, so that a lower member and an upper member functions as a clip. An elastic member is formed in such a manner as to be cut out from an attachment surface of the lower member along the contour (other than a portion thereof) of a closed figure, and can warp with respect to the attachment surface with that portion taken as a warp axis. The elastic member has a projection on the leading end thereof, and a rear panel has a recess in an attachment surface thereof. When the rear panel is rotated with respect to the lower member, the projection is disengaged from the recess by warping of the elastic member, or engaged with the recess by a retroaction of the warping.