A mounting portion structure for stably temporarily fixing a door mirror is provided. Two temporary fixing pieces are formed to protrude on a gasket. Pawls are formed to direct outward at tips of the two temporary fixing pieces. The gasket is previously secured to the mirror base by a screw. The two temporary fixing pieces are inserted through an opening in a mirror mounting portion of a vehicle, and the pawls of the temporary fixing pieces are locked to facing edges of the opening to temporarily fix the mirror base to the mirror mounting portion. Then, the mirror base and the mirror mounting portion are permanently fixed by a screw.