The invention has an object to improve easiness and certainty of the mounting operation of an air bag lid onto an opening of an instrument panel, and upon expanding operation of the air bag, increase the opening area of the air bag lid opening so as not to prevent expansion of the spreading air bag in the passenger compartment. A lid bracket 34 is connected to an air bag lid 15. The air bag lid 15 is secured through the lid bracket 34 to a panel bracket 32 provided on the back of the instrument panel 11 from the surface side of the instrument panel 11. Then, an air bag module body 12 is secured to the panel bracket 32 from the back side of the instrument panel 11. Further, collars 19 pierced with engagement holes 20 on the back thereof are provided on the air bag lid 15. Insertion spaces 36 for insertion of collars 19 and setup claws 37 for engagement in the engagement holes 20 of collars 19 inserted into the insertion spaces 36 are formed in the lid bracket 34.