To eliminate occurrence of the fault of welding part caused by a residual air or metallic vapour of arcing porosity or burn out and blow hole of welding part, etc., by communicating the space through an exhaust passage formed at a fixed interval from the depth part to be electron-beam welded to the surface of a welded member.

On a welding member 2a, a back up member 5 is installed in one body. In a welding member 11a, a fitting space 11h in machine so that it is made in close contact with the welding member 2a and the back up member 5. A through hole 5h as an exhaust passage with the fitting space 11h is formed at a fixed interval L corresponding to the size of the fitting space 11h on the back-up member 5. With this through hole 5h, the fitting space 11h is communicated with a vacuum part outside the welding member with a small air resistance. Therefore, in a butt welding joint with the back up member being able to be positioned precisely, the metallic vapour generated in the electron-beam welding is also sucked immediately to the vacuum and the fault is not generated on the welding part.