1. A taught position modification device for correcting positions of taught points in an operation program of a robot prepared by an offline programming system, using a visual sensor arranged at a movable part of the robot, said device comprising: determining means for determining a position or an orientation of an object of operation based on positions of at least two characteristic points on an image of the object captured by the visual sensor; correction means for correcting the positions of the taught points in the operation program based on the determined position or orientation of the object; moving means for moving an operation tool or a position correction tool attached to an distal of an arm of the robot at the corrected positions of the taught points; setting means for setting a jog-feed coordinate system with respect to the corrected positions of the taught points using information from the offline programming system; and modification means for modifying the corrected positions of the taught point in the operation program based on positions of a control point of the operation tool or the position correction tool, at which positions or orientations of the operation tool or the position correction tool are designated by jog feeds using the jog-feed coordinate system such that the control point takes objective positions for the taught points.