A seam welding system includes a pair of roller electrodes. The pair of roller electrodes hold a to-be-welded object between circumferential surfaces of the roller electrodes, rotate while holding the to-be-welded object between the circumferential surfaces, and perform seam welding on the to-be-welded object when a current flows between the roller electrodes while rotating and holding the to-be-welded object between the circumferential surfaces. To an electrode movement mechanism, the roller electrodes are mounted. The electrode movement mechanism moves the roller electrodes along a welding line of the to-be-welded object. Drive sources respectively rotate the roller electrodes and a joint of the electrode movement mechanism. A controller controls an amount by which the roller electrodes rotate based on a torque change in the drive sources so as to keep torques respectively acting on the roller electrodes within a predetermined range.