PURPOSE: A side panel assembly is provided to reinforce the welded portion rigidity reinforcing structure of a side inner panel and a side outer panel without additional reinforcing panels. CONSTITUTION: A side panel assembly(1) comprises flange portions(F) welded with each other by being composed of a side inner panel and a side outer panel(11). The side outer panel forms an outer curved surface portion(11a) and an outer flanges(F1). The outer flanges are formed for spot welding. The side inner panel comprises an inner curved surface portion and forms space portion(S) with the side outer panel. The side inner panel comprises inner flanges(F2) in the both end portions. The end portions of the inner flange and the outer flange are attached with each other by spot welding. The both sides of the inner flange form reinforcement flanges(F3). The side outer panel and side inner panel is attached by triple stacking spot welding in the both ends.
