To enhance the structure strength of an extension portion that is provided extending to a side part of a seat back body.

A seat back frame 10 for a vehicle seat 1 which includes a seat back body 2A serving as a backrest, and an extension portion 2B which is provided extending to a side part of the seat back body 2A includes: a body frame 10A which forms a framework of the seat back body 2A and is fixed onto a floor; an extension frame 10B connected to a side part of the body frame 10A and forming a framework of the extension portion 2B; and a connection member 16 connecting the extension frame 10B with the body frame 10A. The connection member 16 includes: a rear support member 16A which is fixed to both of the frames 10A, 10B from the rear side and is integrally connected therewith; and a front support member 16B which is fixed to both of the frames 10A, 10B from the front side and is integrally connected therewith.