PURPOSE: To smoothen the assembly work and improve the working efficiency by allowing a CAD control means to select the pattern with which the part installation position is properly displayed and display the pattern on a display screen and carrying out display by superposing the part installation state detected by a detecting member, on the part installation position.

CONSTITUTION: In the anomaly where no part exists or a part is in a tilted state, the position in the coordinate system of the top edge of a robot gun 24 is inputted from a robot controller 21, and coordinate-transformed by a coordinate transformation part 25, and a prescribed position is specified. A CAD control means 27 searches the data for the prescribed position, and the optimum display pattern J for properly the position is selected, and displayed on a display screen 28. When the installation mistake exists, the place of the trouble is color-displayed, e.g., in 'red', by the signal supplied from a sensor 29, and the abnormal state is informed to an operator.