PURPOSE: To save labor and expense for mounting a spoiler in a back door of an automobile.

CONSTITUTION: In a position of a work hole 4A in an inner panel 4 of a back door 2, between inner/outer panels 4, 5, a spacer 11, having an upper surface 11A along the outer panel 5 and a lower surf ace 11B properly opposed to the work hole 4A to arrange a bolt hole 11C in a direction almost orthogonal to the lower surface 11B, is interposed, to insert a bolt 7 of a spoiler 3 to the bolt hole 11C of the spacer 11, and a nut 8 is screwed to the bolt 7 from the work hole 4A of the inner panel 4 of the back door 2.