PURPOSE:To avoid occurrence of a sagging part in the radius and circumference directions of the periphery of a spanner groove when this groove is formed, by almost simultaneously pressing both convex parts provided to the cope and punch and then pressing in sandwich the back by said convex parts. CONSTITUTION:The back 42 is set to the cope 41, and the spanner groove 45 of the 42 is formed by the convex part 44 formed at the punch 43. In this case, the convex part 46 is flatly formed on the cope 41 with nearly similar figures to convex part 44 of the punch through securing some gap in both sides of the inside circumference direction and at least in either side of the radius direction. As a result, the groove 45 of the back is pressed in sandwich between the part 44 and part 46. Accordingly, the peripheral thickness part of the groove 45 is pushed out from inside by the part 46 to thoroughly eliminate the occurrence of a sagging part. As a result, the cutting process to be given later can be excluded to greatly simplify the manufacturing process.