PURPOSE: To form a back relief in which an intermittent shape variation is provided on a borderline of a straight land and a draft taper without exerting no influence at all on the straight land by inserting and attaching a detachable packing material into the straight land of a working hole.

CONSTITUTION: A die 10 in which a working hole 11 consisting of a straight land 12 and a draft taper 13 is formed is brought to blasting with abrasives from the back side of the draft taper 13, and a back relief 14 is formed thereby. Therefore, first of all, a detachable packing material 16 is inserted and attached to the straight land 12 of the working hole 11. Subsequently, from an opening in the back side of the draft taper 13 of the die 10, blasting is executed with abrasives to the draft taper 13 containing a borderline of the straight land 12 and the draft taper 13 by using a blasting device. However, since the packing material 16 is inserted and attached to the inside of the straight land 12, the straight land 12 is not influenced at all thereby, and a back relief 14 in which an intermittent shape variation is provided on the borderline of the straight land and the draft taper 13 is formed.