An inner bracket (40) has a side wall (43) covering the inner side of a back frame (30), a side wall (41) covering the rear side, and a side wall (42) covering the front side. An outer bracket (50) has a support side wall (51) that supports an armrest (20) and covers the outer side of the back frame (30) and a side wall (52) disposed to overlap the side wall (42) of the inner bracket (40). The support side wall (51) is coupled to the side wall (42) of the inner bracket (40) via a bolt (85) and a nut (86). The inner bracket (40) and the outer bracket (50) are both fixed to the back frame (30) by a nut (72) and a through bolt (70) that passes through the side wall (41) of the inner bracket (40), the side wall (52) of the outer bracket (50), the back frame (30), and the side wall (42) of the inner bracket (40).