PURPOSE: To prevent a winding to the side or a bending in the up and down direction in a wavy form of a wire bundle by feeding a current between electrodes only when the wire bundle is seam-welded to the crosspieces of a base member with window holes.

CONSTITUTION: The condition that the rear part 8 of a belt-form base member 7 with window holes is positioned between both roll electrodes 4 and 5 is detected by pilot holes 8a at the left side and the right side of the belt-form base member 7 or by the displacement of the height of the roll electrodes 4 and 5 which have a flat outer surface, and a current is fed intermittently to the rear part 8 and to a wire bundle 2 so as to seam-weld. As a result, the both side edges of the window holes of the belt-form base member 7 never get the current to heat, and the heating of the both side edges is suppressed. Consequently, the resultant multiwire bundle sliding brush elements are never wound to the side nor bent in the up and down direction in a wavy form.