An apparatus for welding of a member to be welded in a vacuum chamber accommodating a part of the member, comprises a housing for defining the vacuum chamber, the housing including upper and lower housings between which the member to be welded is held, the upper and lower housings each having front and rear sides; an electron gun mounted on the upper housing; a drive device for moving the electron gun in a welding direction; an adjustment member or adjustment liner plate disposed on the lower housing for defining spaces allowing a part of the member to be welded to fit in at the front and rear sides of the lower housing so that the entirety of a welding portion of the member is in the vacuum chamber and both sides of the welding line are exposed in vacuum; sealing material for sealing between the housing, the adjustment member or adjustment liner plate and the member to be welded.