Airbag cover with a tear seam

In an airbag apparatus for a passenger seat for use in a vehicle such as an automobile, reinforcement ribs 12 of rectangular horizontal cross-section depend from the underside of a fracture-opening section 30 of an interior cover 3. Mounting holes 14 are formed at predetermined intervals in front and rear sidewalls of the reinforcement ribs 12. Hooks 36 mounted on front and rear side edge surfaces 5b of an airbag case 5 engage the mounting holes 14 via mounting sections 35 extending downwardly from hinge sections 34 of reinforcement plate members 33. Left and right side walls 12b of the reinforcement ribs 12 are spaced from left and right side edges 5c of the airbag case 5 by a predetermined distance, and fracture grooves 30a of the fracture-opening section are formed at positions located within the spacing between the side walls 12b of the reinforcement ribs 12 and the side edges 5c of the airbag case 5.