To provide mounting structure surely mounting an option to a rail groove even with a small operating force.

The mounting structure mounts a mounting device 8 with respect to the rail groove 23. The mounting device 8 comprises: a bracket 81; an operating body 82 disposed to the bracket 81 and receiving the operating force; a pressing part 84 disposed to the bracket 81 and pressing a bottom face 20B and a downward face 234 respectively, and an engagement claw part 83; a fastening mechanism Z1 for making a fastening state Z1 where the rail groove 23 is fastened, by pressing a pressing face 842a and an engagement face 832a against the bottom face 20B and the downward face 234 with the operating force applied to the operating body 82; and a positioning operating body 82b as a width positioning mechanism positioning the bracket 81 to a predetermined position in a width direction of the rail groove 23 in the fastening state Z1.