PURPOSE: To eliminate the need for the painting by forming with pressing a steel sheet having resin films of specific condition with the upper and lower dies whose shoulder radius for forming a cut guiding groove are made in a specific dimension, and forming the cut guiding groove by making the thickness of a machining thinnest part a specific dimension.

CONSTITUTION: A cap body is formed with upper and lower dies 5, 6. The shoulder radius of R parts 8, 9 of cut guiding groove forming parts of the upper and lower dies 5, 6 are made as 0.05 to 1mm. The coating resin of the cap body 1 is made as a crystalline saturated polyester base resin of the thickness 10 to 100μm, the elongation more than 200%, the crystallinity less than 10%, the crystal melting heat more than 10Joul/g. The cap body 1 is formed with pressing, the thickness of steel sheet of the formed thinest part 7 is made less than 40% of the thickness before forming. The circumferential periphery of the opening chip 2 is pressed with dies 10, 11 having projecting parts, and the button and the cut guiding line 4 are formed. Because an acute blade is not used, the coating resin is not flawed.