PURPOSE: To form a titled fixing groove free from the need for an adhesive by press-forming a groove, consisting of a semicircular plane and vertical planes joined to both sides of the plane, in a flat sheet, and squeezing the groove into a dovetail groove.

CONSTITUTION: A groove 14 consisting of a semicircular plane 12 having a desired radius R and vertical planes 13 joined to both sides of the plane 12, is press-formed at the desired position of a metallic flat sheet 11. Next, a lower die punch 15 having a horizontal face 15a at its upper side, is inserted into the groove 14, to mount the flat sheet part 11 on the punch 15. Further, guide dies 16 having each a face 16a parallel to the face 13 are arranged at a desired space interval, and then the top (a) of groove 14 is squeezed by an upper die forming punch 17 having the width larger than that of the groove 14, to form a dovetail groove 18 shown in the figure. Then a packing 19 can be fixed to the groove 18 without using an adhesive, by fitting the square packing 19, whose side width is approximately equal to the width of the horizontal face 12a of groove 18, to the groove 18.