To provide an interior trim fitting structure firm in constitution and excellent in appearance at an A-pillar or the like of an automobile.

The back side wall thickness of a pillar trim 3 is made constant, and a plurality of ribs 8 are formed extending over the whole width. An approximately rhomboid protrusion 12 is integrally formed at the pillar trim 3. A ridge part protrusion 14, a through hole 15 and a cross rib 16 are formed at the approximately rhomboid protrusion 12 of plane shape. Mounting workability is excellent since the approximately rhomboid protrusion 12 is integrally formed, and the ridge part protrusion 14 and the through hole 15 facilitate insertion and suppress slip-off. Further, there is no excessive play since the approximately rhomboid protrusion 12 and the end part of the cross rib 16 abut on the inner peripheral surface of an insert hole of a pillar inner panel 6, and appearance is improved since the pillar trim 3 is not distorted by molding.