Abstract: First longitudinal grooves are formed in the lateral sides of wing parts of a cap and a slider. First through holes are formed to intersect the first longitudinal grooves in the cap and the slider. Second through holes are formed in positions closer to the centers of the cap and the slider than the first through holes. In the cap, two ribs pointing in the lateral directions are individually supported to be pivotable at the first longitudinal grooves by supporting members in the first through holes, and other ribs are pivotally supported in a collective manner by a circular supporting wire passing through the second through holes. In the slider, two stretchers pointing in the lateral directions are individually supported to be pivotable at the first longitudinal grooves by supporting members in the first through holes, and other stretchers are pivotally supported in a collective manner by a circular supporting wire passing through the second through holes.