A one-side arc welding device (100) capable of performing welding so as to prevent occurrence of a welding defect comprises a backing member (21, 22) which extends in a welding direction, magnetic attraction members (27-29) which are arranged in the longitudinal direction of the backing member at the side of the backing member and magnetically attract a steel plate to be welded (1, 2) located on the backing member, a welding torch (4) which moves along a welding line (3) of the steel plate to be welded and sends out a welding wire (5), and a welding power source (6) which supplies an electric current to the welding wire to generate an arc between the welding wire and the steel plate to be welded. The welding power source is installed on the welding start side or welding end side of the welding line, connects each of the magnetic attraction members and a power source ground terminal (6b) of the welding power source via a power source ground connection terminal (101a) installed on the welding start side of a stand frame (101), and a ground current returns to the welding power source via the plurality of magnetic attraction members and the welding start side of the stand frame disposed from the welding start side to the welding end side of the welding line.