A vibration welding method, a vibration-welded structure, and an airbag apparatus capable of reducing the positional deviation of junction between workpieces different in the die releasing direction are provided. An airbag apparatus according to one form of the present invention includes an airbag that is normally folded and is inflated and developed at an emergency; an inflator for supplying gas to the airbag; a retainer for accommodating the airbag and the inflator therein; an outer panel mounted on an instrument panel; and an inner case vibration-welded to the inside of the outer panel for retaining the retainer, in which when a die releasing direction of the outer panel differs from a die releasing direction of the inner case, a positioning plane of the inner case is molded to agree with the die releasing direction of the outer panel while the outer panel is provided with a positioning rib formed thereon.