A coupling member includes: first, second, and third portions each having a flat plate-like shape; and a fillet portion. The first portion has a first side and a second side orthogonal to each other. The second portion is bent at the first side relative to the first portion, and extends to an inner surface of the first portion. The third portion is bent at the second side relative to the first portion, and extends to an inner surface of the first portion. A bend radius of an outer surface of the coupling member at the first side is equal to a bend radius of a surface of a first extending member facing the first side. A bend radius of an outer surface of the coupling member at the second side is equal to a bend radius of a surface of a second extending member facing the second side.