To provide a mounting structure for easily and surely mounting an exterior wall material to a beam.

When a supporting bolt (7) on the beam (1) side is inserted in a slit (22) of the horizontal member (19) while moving the exterior wall material (11) toward in the inside direction of a building from the outside of the building, a hook (25) crossing the slit (22) is pushed against the supporting bolt (7) to rotate up to a slit opening position, and when the supporting bolt (7) is inserted up to the depth side of the slit (22), the hook (25) is returned to a slit crossing position, and the supporting bolt (7) on the beam (1) side is engaged with the horizontal member (19) on the exterior wall material (11) side. Accordingly, the exterior wall material (11) can be surely mounted on the beam (1) at one touch.