PURPOSE: To improve production efficiency by a method wherein after a body side panel and the side frame are clamped, a joint therebetween is welded to form an assembly, the assembly is laterally rotated as it is to hold it in an actual car state, and with this state, other part is welded.

CONSTITUTION: Simultaneously with clamping of the upper part of a wheel apron W1 and a first clamp device 10, the lower part of the wheel apron and a side frame W2 are clamped in a state that coupling flange parts W1f and W2f are brought into contact with each other. Under this state, a joint is temporarily welded by a spot welder to form a subassembly W. After a base 41 and a revolving unit 30 are raised to a given position in a state that the subassembly W is clamped, the revolving unit 30 is rotated approximately in a 90° arc and brought into an actual car state. After a pair of the subassemblies W on both sides are set and brought into an actual car state by means of body assembling devices 1 on both sides, front and rear parts are assembled to the front and rear of each of the subassemblies W on both sides.