To provide a method of assembling a flat display device in which occurrence of displacement between two panels can be prevented effectively despite of a simplified constitution.

In the method of assembling the flat display device, a first panel P1 and a second panel P2 are aligned in a state that a joint member 30 is arranged in their peripheral parts, and subsequently after positioning of the second panel P2 to the first panel P1 is carried out, while a force proceeding to a first ridge extending in a first direction of the second panel P2, a force proceeding to a second ridge extending in the second direction, and a force proceeding to the first panel P1 are added against the second panel P2 by an energizing means, and by heating the joint member 30, the first panel P1 and the second panel P2 are joined at their peripheries.