A metal strip 10 is bent to form a ring and end parts 12 and 14 are connected to each other. An electron beam 26 may be defocused and emitted to a welded section 20 along a crosswise direction of the metal strip 10. Next, a focal point 28 of the electron beam 26 may be focused onto a weld-melted portion 25 to execute electron beam welding. Subsequently, the electron beam 26 may be defocused emitted to the welded section 20 along the crosswise direction of the metal strip 10, and the welded section 20 may be further cooled. The average of the dendrite secondary arm spacing of the weld-melted portion may fall within a range of 7 to 30 μm.