FIGS. 4A and 4B are a side view and a cross section showing a state in which a slider has been mounted on the slider base portion11, whereas FIG. 6 is a perspective view showing a slider.A slider 31 is made up of a combination of an inner slider 32 and an outer slider 33; the inner slider 32 is inserted into the outer slider 33 from the side of the outer slider 33.