A welding device (1) performs one-side submerged arc welding of a steel plate by using a first welding unit (10) which uses a first wire (110), a second welding unit (20) which uses a second wire (120), a third welding unit (30) which uses a third wire (130), and a fourth welding unit (40) which uses a fourth wire (140). When doing so, the first welding unit (10) controls the feeding speed of the first wire (110) so as to be a constant speed by using a first feeding device (11), and supplies power to the first wire (110) by using a first welding power source (12) for which the power-supply method is set to DC and the external properties are set to a constant voltage. As a result, it is possible to reduce faults in the appearance of a penetration bead obtained by the one-side submerged arc welding of a multielectrode.