To prevent a car body deformation by a load input at the time of a side surface collision from occurring by increasing the strength of a combined section so that a bracket rail side and a brace center can be combined in such a manner that a bead terminal formed at the brace center and the bracket rail side may surely form a closed cross section when both parts are combined.

An extension member 3 which is extended into the cabin side from a side rail in order to house an upper arm of a sliding door is formed to have a width larger than the width of the brace center. At the same time, recesses 4a and 4b which are formed on the extension member and the brace center 4 are combined in a manner to form the closed cross section.