An amorphous structure layer is formed on a surface layer of a bonded portion of each of side brackets. A bottomed hole layer including a plurality of bottomed holes is formed on a surface layer of the amorphous structure layer. Each of the bottomed holes has a reverse-tapered shape, which has, between an opening portion and a bottom portion of each of the bottomed holes, a bulged portion having a larger inner circumference than the opening portion. An adhesive is injected into the bottomed holes. An outer circumferential surface of the bonded portion of each of the side brackets and an inner circumferential surface of an end portion of a center beam face toward each other with the adhesive interposed therebetween.

An amorphous structure layer is formed on a surface layer of a bonded portion of each of side brackets. A bottomed hole layer including a plurality of bottomed holes is formed on a surface layer of the amorphous structure layer. Each of the bottomed holes has a reverse-tapered shape, which has, between an opening portion and a bottom portion of each of the bottomed holes, a bulged portion having a larger inner circumference than the opening portion. An adhesive is injected into the bottomed holes. An outer circumferential surface of the bonded portion of each of the side brackets and an inner circumferential surface of an end portion of a center beam face toward each other with the adhesive interposed therebetween.