A robot controller that controls a tandem arc welding system according to the present invention includes a leading-electrode correcting section that calculates a leading-electrode correction amount, used for correcting a displacement in a left-right direction and an up-down direction, from a leading-electrode changing amount calculated by a leading-electrode processing section; a leading-electrode correcting section that calculates a trailed-electrode correction amount, used for correcting a displacement in a rotational direction, from a trailed-electrode changing amount calculated by a trailed-electrode processing section; a rotational-displacement correction controlling processing section that calculates a rotational-center correction amount for correcting the displacement of the leading electrode; and a robot trajectory planning processing section that corrects a teaching position and a position of a rotational center of a welding torch during tracking correction. By such a structure, even if arc tracking is carried out at any rotational center, displacement of the leading electrode does not occur, so that defective welding does not occur.