Abstract: The polyester resin composition of the present invention contains a polyethylene terephthalate resin (A) and a deflection temperature modifier (B). The polyethylene terephthalate resin (A) is crystallizable, and when the polyethylene terephthalate resin is in a crystallized state, the polyester resin composition has a minimum tensile strength of 5 MPa or less in a temperature range of 150° C. to 220° C., and has a tensile elongation of 50% or more in this temperature range.

Abstract: A reversed U-shaped base frame for a seatback frame structure is of a monoblock structure, which includes an upper horizontal tubular portion and two side vertical portions. The two side vertical portions extend downward from axially opposed ends of the upper horizontal tubular portion. The base frame is produced from a shaped metal sheet by pressing and curling the same in such a manner that a given portion of the shaped metal sheet, which is shaped to produce the upper horizontal tubular portion, is curled to have a substantially circular cross section. The feature of the invention is that the curled given portion has circumferentially opposed edges which are overlapped each other.