A side plate of a casing includes bracket members and a surface member. Each of the bracket members includes three second coupling portions and a plate-shaped attachment portion provided integrally with the second coupling portions. A coupling pin is inserted into the second coupling portions. The surface member is joined to the plate-shaped attachment portion. The bracket member includes a step fitted to an end of the surface member. The plate-shaped attachment portion and the surface member are jointed at a spot welding section including spots in a zigzag pattern.