To provide a mounting structure of a light source unit capable of being easily mounted to a vehicle interior material.

The mounting structure of a light source unit 60 to a vehicle door trim 10 includes: a LED 61; a light guide body 63 having an end surface 63A, into which light from the LED 61 is input, and an outer peripheral surface 63D which outputs the light input from the end surface 63A; a LED housing member 71 for housing the LED 61; a light guide body holding member 81 for holding the end of the light guide body 63 including the end surface 63A; and a pair of mounted ribs 90, which are arranged in a vehicle door trim 10 and to which the LED housing member 71 and the light guide body holding member 81 are individually mounted. The LED housing member 71 and the light guide body holding member 81 are connectable to each other. By connecting the LED housing member 71 to the light guide body holding member 81, the LED 61 and the end surface 63A are arranged to face each other.