A seat back frame including an upper frame forming an upper framework of a seat back and extending in a seat width direction, and a side frame forming a side framework of the seat back and extending in a seat height direction, wherein an upper end of the side frame extends in the seat height direction up to a portion of the upper frame extending in the seat width direction, and wherein the side frame includes a general surface portion which is in contact with the upper fame from below with its surface facing the seat width direction, and a flange portion which protrudes from the general surface portion with its surface facing a seat front and rear direction, is in contact with the upper frame from at least one of the front and rear sides, and is welded to the upper frame.