To make an upper end portion of a lower rod bracket compact while ensuring reliability of high strength of a fastening portion of a side rail.

In a mounting structure of a side rail 1 of a lower rod bracket 6, an upper end portion of which is attached to an outer side surface of a web of the side rail 1 to extend downwardly and a lower end portion of which supports a lower rod, the upper end portion of the lower rod bracket 6 and an end portion of a cross member 16 provided on an inner side of the web of the side rail 1 are jointly fastened by a plurality of bolts 13. At least one of the bolts 13 is a taper bolt 13'(a bolt with a neck portion). The upper end portion of the lower rod bracket 6 and the end portion of the cross member 16 are mechanically tied up by the taper bolt 13' to be integrally slidable relative to a surface of the web of the side rail 1.