To provide the welded structure, which is lightweight and is manufactured easily, of a boss for a working-machine arm.

The first holes 14 and the second hole 15 in fixed shapes are formed to a cylindrical body 11 formed of one thin sheet metal. Seat plates 23 for the bosses covering the first holes 14 are installed to the bosses 20, and a seat plate 32 for a bracket covering the second hole 15 is mounted on the bracket 30. The bosses 20 and the bracket 30 are inserted to the cylindrical body 11, the first holes 14 are covered with the seat plates 23 for the bosses from the inside, and the second hole 15 is covered with the bracket seat plate 32 and the bosses and the bracket are welded to the cylindrical body 11 from the outside of the cylindrical body 11 respectively.