Abstract: A device directed to an incomplete seat sub-assembly having a foam cushion member and a trim cover assembly fixed thereto, extending a reversely turned overhang section therefrom. The device basically includes a frame assembly including a support frame having a configuration generally equal in shape to a part of outer peripheral sides of the cushion member, on which support frame, the incomplete seat sub-assembly is turned upside down and placed, a cramp element for securing an end of the overhang section of trim cover assembly to the support frame, and a pressing element for applying a downward pressure towards the support frame, so that the cushion member is pressed and moved downwardly by the pressing element, while the overhang section is turned over relative to the support frame and cover a whole body of the cushion member.