PURPOSE: To reproduce an action reflecting an actual condition by performing luminous dotting in a welding means after mutual positioning of an actual assembly part in an installing position when an assembling process of assembling parts is dynamically exhibited in an actual dimension.

CONSTITUTION: In virtual assembly, an under body 1 is supported by means of a jig piece 18 in the first jig unit 11, while left and right side bodies 2 are supported by means of two jig piece 25 on left and right sides in the second jig unit previously. Subsequently, a loop 3, a cowling 4, and an upper back 5 are supported by means of jig pieces (a hanger 32 and a vacuum cup 33) in the third jig unit 13, while a lower back 6 is supported by means of a jig piece 46 in the fourth jig unit 14 respectively, and then, positioning to the waiting position is carried out. When an assembly command is outputted, respective parts are positioned in the predetermined positions, and then, welding machines 71-73 are started according to an outputted welding command, and a lighting part, of a vertual welding gun is lighted (luminous dotting), and consequently, assembly of a main body is finished virtually.