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 in annular shape disposed between the facing surfaces , the ring gear including internal teeth on an entire circumference of an inner peripheral surface and external teeth on at least a part of an outer peripheral surface, the ring gear 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 engaged with the internal teeth of the ring gear ; 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 ; a ring output gear coupled to the welding head turning gear , the ring output gear engaged with the external teeth ; 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 , wherein the welding head turning gear and the ring gear 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 in annular shape disposed between the facing surfaces , the ring gear including internal teeth on an entire circumference of an inner peripheral surface and external teeth on at least a part of an outer peripheral surface, the ring gear 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 engaged with the internal teeth of the ring gear ; 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 ; a ring output gear coupled to the welding head turning gear , the ring output gear engaged with the external teeth ; 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 , wherein the welding head turning gear and the ring gear are rotatably supported with respect to the housing by a guide member with respect to the guide surface. <EOS>