PURPOSE: To efficiently cut off the plate of closing belt to be collected for a spherical tank, by fixing a cutting jig, to which a rail is mounted, to a predetermined position on a sheet material and allowing a self-propelled saw to run on the rail.

CONSTITUTION: A sheet material 1 of aluminum is placed on a wooden bed 2. A cutting jig 5, to which a self-propelled saw 3 and a running rail 4 of the self- propelled saw are mounted, is fixed to a predetermined position on the material by an I-bolt 6 and a hanging metal fixture 8 and clamps 12. The sheet material 1 of aluminum is cut off by allowing the self-propelled saw 3 to run on the rail 4 further rotating a saw tooth 3A of the saw 3. Here the speed of cutting is suitably selected by a thickness or the like of the sheet material 1 of aluminum but set to a speed of about 1m/min.