To miniturize a support bracket, by performing tightening work with a tool without being restricted by a roof rack rail.

In this mounting structure, a support bracket 35 mounting a front/rear intermediate part 5 of a roof rack rail 1 in a roof panel 3 is provided, the support bracket 35 is divided into two parts, to be formed as a roof side bracket part 41 mounted in a side of the roof panel 3 and a rack side bracket part 43 mounted in the front/rear intermediate part 5 of the roof rack rail 1, the roof side bracket part 41 is formed by a base part 45 placed on the roof panel 3 and a roof side connection part 47 standing inside a car width direction of the base part 45. In the rack side bracket part 43, a rack side connection part 57 protruded to a lower side of the roof rack rail 1 is provided, the base part 45 of the roof side bracket part 41 is fixedly tightened to the roof panel 3 by tightening fixtures 63, 71, in the roof side connection part 47, the rack side connection part 57 is superposed together from a car width direction, to be fixedly tightened by tightening fixtures 49, 65.