A frame structure to be provided in a seat back having a seatbelt retractor and a seatbelt guide at a top of a seat, wherein the frame structure is an integral structure including an oblique frame obliquely extending upwardly to a seatbelt guide installation section from a first support point located at an end side of a lower part of the seat back in a seat width direction, a vertical frame extending upwardly from a second support point located at another end side of the lower part of the seat back in the seat width direction and connects to the oblique frame at a top, and a transverse frame extending between a lower end of the oblique frame and a lower end of the vertical frame.