In a joining structure, a protruding portion on the side of a first component is a hollow body that prevents bringing about sinking, deforming, and cracking of the side surface of the first component part, even in the event that the protruding portion is enlarged in order to increase strength for connecting the first component part and a second component part. In addition, the connecting groove on the side of the second component part is also defined by a hollow body that includes a pair of supporting ribs and a supporting plate bridging the pair of supporting ribs. Consequently, the hollow body can prevent defects of sinking, deforming, and cracking in the side surface of the second component part, even in the event that the supporting ribs and the supporting plate are increased in thickness in order to enhance the rigidity of the coupling arrangement between the first and second component parts.