A teaching device and a teaching modification device capable of easily attaining conformity between an operation program of a robot prepared by off-line programming and an actual operation of the robot. A layout of a robot system including three-dimensional models of the robot and peripheral objects thereof (table, a workpiece, etc.) are prepared by an off-line programming system and taught points are defined for the workpiece. The system layout and a model of the workpiece are displayed on a display device of a teaching pendant. An operator specifies a present position of the operator in the system layout and a taught point to be modified referring to the display device. A line-of-sight vector is automatically calculated and the model of the workpiece as viewed from a direction of the line-of-sight is displayed on the display device. Thus, the operator can operate the robot for modifying positions of the taught points or orientations at the taught points with ease, referring to the three-dimensional model of the workpiece and the taught points on the display device as viewed from the present position of the operator.