A data processing apparatus for processing data described in a welding operation program of an arc welding robot system. The data processing apparatus includes a data obtaining section for obtaining a plurality of position and orientation data at a plurality of different teaching points previously taught and included in the welding operation program; a reference plane setting section for setting a virtual reference plane as a reference defining a geometric placement of an arc welding torch relative to a workpiece during a welding operation, based on the position and orientation data obtained in the data obtaining section; an angle calculating section for calculating a plurality of angle data representing the geometric placement of the arc welding torch at every teaching points, by using the position and orientation data obtained in the data obtaining section and the virtual reference plane set in the reference plane setting section; and an angle processing section for performing at least one of a displaying process and a correcting process of the angle data calculated in the angle calculating section.