1. An air bag mounting interior trim comprising:a retainer having mounted thereto a case housing an air bag, the retainer having a first wall portion disposed at a deployment side with respect to the air bag; and
a panel having a second wall portion disposed so as to be opposed to the first wall portion of the retainer, at the deployment side of the air bag with respect to the first wall portion, the panel configured to be welded to the retainer, wherein
the first wall portion and the second wall portion each have an inclined face inclined with respect to a pressurizing direction in which the retainer and the panel are moved relative to each other at a time of assembly thereof for welding,
the retainer has a first rib protruding along the pressurizing direction from the inclined face of the first wall portion toward the panel,
the panel has a second rib configured to be welded to the first rib, the second rib protruding along the pressurizing direction from the inclined face of the second wall portion toward the retainer, and
the first rib and the second rib respectively have a same width and weld end faces, the weld end faces of the first and second ribs face each other, are parallel to each other and extend orthogonally relative to the pressurizing direction; and the weld end faces of the first and second ribs have matching contact surface areas and are welded together without one of the weld end faces surrounding another of the weld end faces.