Thermal strain due to welding occurs when long materials having a U-shaped cross section are joined together. The joining work is complicated. To an end portion of a first groove-shaped member having a shape in which side surface portions rise from both ends of a bottom surface portion, that is, a substantially U-shaped cross section, an end portion opposing thereto of a second groove-shaped member having the same shape is fitted. Their bottom surface portions and their side surface portions are kept in close contact with each other. A bottom surface clinched portion is formed on the bottom surface portions in the fitted portions by spot clinching. A pair of edge folding portions are formed on each of the pairs of the side surface portions by hemming bending. Furthermore, a side surface clinched portion is formed on each of the edge folding portions by spot clinching. These three kinds of processing are performed in a single processing step of metal dies. The joining strength is increased and occurrence of thermal strain is eliminated, and efficient joining work can be performed.