To improve the supporting rigidity and durability by providing a longitudinal extension part fastened to a side member and a cross directional extension part fastened to a side seal inner.

A cross member bush is fitted on the periphery of a bolt so as to be interposed between a lower stopper 50 and the side member, and a bolt hole 90 for insertion of the bolt is provided at a central part of the lower stopper 50. A longitudinal extension part 58 of the lower stopper 50 is fastened to the side member 2 by bolts at two positions so as to reinforce the rigidity of a car body in the fore and aft direction. The longitudinal extension part 58 is provided with two bolt holes 80 for insertion of the bolts with the predetermined space in the cross direction. A cross directional extension part 60 of the lower stopper 50 is fastened to the side seal inner by bolts so as to reinforce the rigidity of the car body in the right and left direction, and the cross directional extension part is provided with a bolt hole 84 for insertion of a bolt.