A welding apparatus comprising: a housing in annular shape having two facing surfaces in an axial direction and a guide groove continuous in a circumferential direction formed on each of the facing surfaces; a welding head turning gear in annular shape disposed between the facing surfaces, the welding head turning gear including internal teeth on an entire circumference of an inner peripheral surface; at least one ring gear (50, 60, 70) in annular shape disposed between the facing surfaces, the ring gear (50, 60, 70) including internal teeth (58, 68, 78) on an entire circumference of an inner peripheral surface and external teeth (59, 69, 79) on at least a part of an outer peripheral surface, the ring gear (50, 60, 70) pivotably provided with respect to the welding head turning gear at more than or equal to a predetermined angle; a turning input gear engaged with the internal teeth of the welding head turning gear and a ring input gear (52, 62, 72) engaged with the internal teeth (58, 68, 78) of the ring gear (50, 60, 70); a plurality of electric motors (M1-M4) coupled to the housing so as to be positioned at one side in the axial direction with respect to the housing, the plurality of electric motors (M1-M4) configured to transmit torque to the turning input gear and the ring input gear (52, 62, 72); a ring output gear (54, 64, 74) coupled to the welding head turning gear, the ring output gear engaged with the external teeth (59, 69, 79); and a welding head having a welding torch positioned at an other side in the axial direction with respect to the housing, the welding head having a torque use means to which torque is transmitted from the ring output gear (54, 64, 74), wherein the welding head turning gear and the ring gear (50, 60, 70) are rotatably supported with respect to the housing by a guide member with respect to the guide surface. a housing in annular shape having two facing surfaces in an axial direction and a guide groove continuous in a circumferential direction formed on each of the facing surfaces; a welding head turning gear in annular shape disposed between the facing surfaces, the welding head turning gear including internal teeth on an entire circumference of an inner peripheral surface; at least one ring gear (50, 60, 70) in annular shape disposed between the facing surfaces, the ring gear (50, 60, 70) including internal teeth (58, 68, 78) on an entire circumference of an inner peripheral surface and external teeth (59, 69, 79) on at least a part of an outer peripheral surface, the ring gear (50, 60, 70) pivotably provided with respect to the welding head turning gear at more than or equal to a predetermined angle; a turning input gear engaged with the internal teeth of the welding head turning gear and a ring input gear (52, 62, 72) engaged with the internal teeth (58, 68, 78) of the ring gear (50, 60, 70); a plurality of electric motors (M1-M4) coupled to the housing so as to be positioned at one side in the axial direction with respect to the housing, the plurality of electric motors (M1-M4) configured to transmit torque to the turning input gear and the ring input gear (52, 62, 72); a ring output gear (54, 64, 74) coupled to the welding head turning gear, the ring output gear engaged with the external teeth (59, 69, 79); and a welding head having a welding torch positioned at an other side in the axial direction with respect to the housing, the welding head having a torque use means to which torque is transmitted from the ring output gear (54, 64, 74), wherein the welding head turning gear and the ring gear (50, 60, 70) are rotatably supported with respect to the housing by a guide member with respect to the guide surface.