To provide a mounting structure capable of preventing the improper operation of a switch for opening a tailgate door fitted to a garnish even if a collision load is applied from a rear part of a vehicle.

In the mounting structure of a switch 10 for a tailgate door which is fitted to a garnish 6 fitted to the tailgate door 2 and electrically opening a lock mechanism for holding the tailgate door on a vehicle body side in a closed state, an operation unit 10B of the switch 10 is located in an opening 9 of a holding part 8 formed on the garnish 6, and fitted to the garnish via a spacer 11 between a switch body 10A and the garnish.