To improve shape freezing accuracy between a floor main and a body side joined by welding in a body combining/a temporary stapling process for the first time.

Of each joining surface of the floor main Fm and the body side Bs, the accuracy of at least one measuring point 17A to 17K about each of eleven welding joint portions 12A to 12K is previously measured in a component discrete state. On the basis of these measured data, generation of interference or an clearance at the welding joint portions 12A to 12K when both the floor main Fm and the body side Bs are supported to be actually faced and joined. The part supported not to generate the interference and the clearance is welded first. In that case, a part having large stiffness is made to be high priority about the precedence order of welding.