A seat back frame includes a main frame that forms a frame of a seat back of a main seat, and an auxiliary frame that forms a frame of a seat back of an auxiliary seat adjacent to the main seat in a width direction, and that is integrally formed with the main frame. The main frame has a first open portion that opens toward a side with the auxiliary seat in the width direction. An end portion of the auxiliary frame is a fitting structure that fits together with the first open portion, and a portion of the fitting structure is welded.