Described herein is a slag removing apparatus suitable for use on a padi-welding machine, which permits automation of the pad-welding operation and which essentially includes: a C-shaped roller holder located beneath a movable welding torch and provided with a rotational drive mechanism for a roller to be padded; a pair of extension arms horizontally extended on the open side of the roller holder and fixed in vertically adjustable positions; a guide rail support member provided with a rack and bridged between the extension arms in parallel relation with the rotational axis of the roller drive mechanism and detachably at least at one end thereof; a carriage having on a rockable plate a drive motor and a pinion disengageably meshed with the rack and movable along the support member; an inclined mount plate provided on the carriage with a predetermined angle of inclination; and a vibrational beating mechanism holding at least one needle bundle with the fore end thereof in face-to-face relation with the welding surface of the roller; the drive motor being driven in synchronism with movements of the torch in the transverse direction.