A positioner for arc welding having a few movable members and a simple structure, an arc welding robot system including the positioner and an arc welding robot, whereby the orientation of the workpiece corresponding to a welding site of the workpiece may be suitably changed by a minimum motion of the positioner. The positioner includes a pair of support members, a first member having two ends supported by the pair of support members such that the first member is rotatable about a first axis parallel to an installation surface of the positioner, a second member having two ends supported by a pair of supporting portions arranged on the first member such that the second member is rotatable about a second axis perpendicular to the first axis, and a workpiece fixing part for fixing the workpiece to the second member.