Abstract: A first retainer engages with an exterior side of an interior wall member to retain a lamp unit on an interior side of the interior wall member. A second retainer engages with an exterior side of a panel body to retain the interior wall member, on which the lamp unit is mounted, on an interior side of the panel body. The second retainer includes a flexible arm having a free end portion which is flexible in a first direction parallel with the panel body and a second direction perpendicular to the panel body. The free end portion is abutted against the exterior side of the panel body when the interior wall member is mounted on the panel body. A regulator prevents the free end portion of the flexible arm from being flexed more than a predetermined amount in the second direction when the flexible arm is not flexed in the first direction.