To provide a seat frame equipped with a seat back frame which is improved in rigidity over tension of a seatbelt with a comparatively simple structure.

The seat back frame includes: a first side member and a second side member which are long and extend in the vertical direction of the seat back to support both sides of the seat back. The first side member is configured from a single steel plate comprising: a side member sidewall part disposed parallel to the direction in which the tensile force of the seatbelt acts on a seatbelt guide part and continuously formed in the longitudinal direction of the first side member; a side member front wall part formed forward of the side member sidewall part relative to the vehicle; and a side member rear wall part formed rearward of the side member sidewall part relative to the vehicle. Also its cross-sectional shape is an open cross-sectional shape.