PURPOSE: To prevent an air bag for a front passenger's driver's seat from being broken when the air bag is operated.

CONSTITUTION: An installation piece 26 is projectingly installed on the back surface of a lid 16, and a tear line 30 is formed at the root part of the installation piece 26. The tear line 30 has not only a groove but a plurality of holes 42 which penetrate through the thickness direction of the installation piece 26. Accordingly, when the lid 16 is opened, being pushed by an air bag, the installation piece 26 is torn along the tear line 30. Since the tear position is the position set along the back surface of the lid 16, the rubbing between the developed air bag and the torn surface is extremely less. Since the smooth surface of the hole 42 exists on the torn surface, the air bag is surely prevented from being broken.