To unite a sub member to a main member in an inexpensive structure without lowing yield strength of the main member.

A sub lower 221 of a sub member 22 comprises: a first sub lower 222 including one part of a side wall adjacent to a main member 21 and one part of a bottom wall; and a second sub lower 223 including the other parts of the side wall and the bottom wall of the sub lower 221, and thereby uniting the first sub lower 222 and the second sub lower 223 to form the sub lower 221. The first sub lower 222 is welded and connected to the main member 21 to form an acute angle therebetween, and then the side wall and the bottom wall lapped between the first sub lower 222 and the second sub lower 223 are welded and connected, thereby uniting the first sub lower 222 and the second sub lower 223, and also the second sub lower 223 is welded and connected to the main member 21.