Cap mounting structure

A cap mounting structure has an engagement hole formed in a peripheral edge part of an opening in an interior member; a protruding part is formed on a cap; an engagement part engaging with the engagement hole is formed by expanding an intermediate portion in the protrusion direction of the protruding part; the length in the expansion direction of the engagement part is set so as to be longer than the length of the engagement hole; the engagement part is inserted through the engagement hole while at least one of the engagement hole and the engagement part is elastically deformed, and the engagement part is engaged with the engagement hole by the elastic restoration of the elastically deformed part(s), whereby the cap is made in a temporarily held state in which the cap is held by the interior member; and from the temporarily held state, the cap is oscillated so as to be opened and closed.