PURPOSE: To fix the termination of skin and a carpet to the back surface of a frame of a seat back without impairing its appearance and quality, wherein the frame of seat back is made through blow molding.

CONSTITUTION: When a carpet 2 is set in a blow molding die 8 for a frame of seat back, the end 21, of the carpet 2 is provisionally fixed using a slit plate 5 to a magnet 6 embedded in the die 8, and then the frame 1 is formed through a blow molding process, and the termination of a skin is secured to the part formed as recessed by the slit plate 5, and this termination of skin is covered with the end 21 of the carpet 2 which is not fusion attached to the frame 1.