Abstract: According to the present invention, n (n=S is a positive integer) pieces of interrupting grids (22-1, 22-2, 22-3, - - - , 22-(n-1) and 22-n) are provided, each of the interrupting grids encompasses P pieces of narrow cut-off canals arranged in parallel. The both side of each of the narrow cut-off canals are concaved so that the narrow cut-off canal has a waisted-mortar shape. The waisted-mortar shape is delineated by m pieces of elliptical holes (Q1, Q2, Q3, - - - , Qm-1 and Qm (m=P?1 is a positive integer)) arranged adjacently in parallel and semi-elliptical holes (cut ID portions) provided on both sides the alignment of elliptical holes. Then, n pieces of the interrupting grids 22-1, 22-2, 22-3, - - - , 22-(n-1) and 22-n are arranged in series through jointing zones (heat-radiation zones) (21-1, 21-2, 21-3, - - - , 21-(n-) and 21-n) having a length of 2.5 millimeters or less measured in the series direction.