A seam welding system (1, 1'), characterized by: a pair of roller electrodes (26, 27) configured to hold a to-be-welded object between circumferential surfaces of the pair of roller electrodes (26, 27), configured to rotate while holding the to-be-welded object between the circumferential surfaces, and configured to perform seam welding on the to-be-welded object when a current flows between the pair of roller electrodes (26, 27) while the pair of roller electrodes (26, 27) are rotating and holding the to-be-welded object between the circumferential surfaces; an electrode movement mechanism to which the pair of roller electrodes (26, 27) are mounted, the electrode movement mechanism being configured to move the pair of roller electrodes (26, 27) along a welding line of the to-be-welded object; a plurality of drive sources (M1, M2, M3, M4, M5, M6, M7, M8) respectively configured to rotate the pair of roller electrodes (26, 27) and a joint of the electrode movement mechanism; characterized in that servo motors (Ml to M8) are provided, which are drive sources to rotate the pair of roller electrodes 26 and 27 and the joints of the robot arm 13, and wherein a torque change is acquired based on torque commands fed back from the servo motors (Ml to M8) in a sensor-less manner, wherein the seam welding system further comprises a controller configured to control an amount by which the pair of roller electrodes (26, 27) rotate based on the torque change in the plurality of drive sources (M1, M2, M3, M4, M5, M6, M7, M8) so as to keep torques respectively acting on the pair of roller electrodes (26, 27) within a predetermined range. a pair of roller electrodes (26, 27) configured to hold a to-be-welded object between circumferential surfaces of the pair of roller electrodes (26, 27), configured to rotate while holding the to-be-welded object between the circumferential surfaces, and configured to perform seam welding on the to-be-welded object when a current flows between the pair of roller electrodes (26, 27) while the pair of roller electrodes (26, 27) are rotating and holding the to-be-welded object between the circumferential surfaces; an electrode movement mechanism to which the pair of roller electrodes (26, 27) are mounted, the electrode movement mechanism being configured to move the pair of roller electrodes (26, 27) along a welding line of the to-be-welded object; a plurality of drive sources (M1, M2, M3, M4, M5, M6, M7, M8) respectively configured to rotate the pair of roller electrodes (26, 27) and a joint of the electrode movement mechanism; servo motors (Ml to M8) are provided, which are drive sources to rotate the pair of roller electrodes 26 and 27 and the joints of the robot arm 13, and wherein a torque change is acquired based on torque commands fed back from the servo motors (Ml to M8) in a sensor-less manner, wherein the seam welding system further comprises a controller configured to control an amount by which the pair of roller electrodes (26, 27) rotate based on the torque change in the plurality of drive sources (M1, M2, M3, M4, M5, M6, M7, M8) so as to keep torques respectively acting on the pair of roller electrodes (26, 27) within a predetermined range.