According to such a configuration as described above, when the doubled yarn 104 is stretched so as to bridge between the grooves 115 and 115 of the side plates 112 and 112, the doubled yarn 104 comes into contact with the edge 117 of the inclined surface 116 so that the constituent yarns 104a and 104b are juxtaposed widthwise of the groove 115 along the edge 117.