PURPOSE: To enhance productivity by mounting a seat cover attached in a turned-out state so as to cover a cushion pad while reversing the same by allowing the cushion pad to penetrate into the arm group of a seat cover holding mechanism.

CONSTITUTION: A turned-out seat cover 1 is mounted on a seat cover holding mechanism 60 holding a cushion pad 2 to a cushion pad holding mechanism 80 at a predetermined position and equipped with a plurality of arm groups extending along the direction becoming the up-and-down direction of a seat back and placed at the position surrounding the cushion pad 2 from the outside so as to cover the arm groups. The seat cover holding mechanism 60 and the cushion pad holding mechanism 80 are relatively moved by a drive mechanism and the cushion pad 2 is allowed to penetrate into the arm groups of the sheet cover holding mechanism 60 to reverse the turned-out seat cover 1 to cover the cushion pad 2.