PURPOSE: To realize smooth development of an air bag without generating any swelling on the front surface side, and giving a good appearance, by forming a hollow part integrally inside a cover body which is formed of a thermoplastic resin, so as to compose a breaking part.

CONSTITUTION: On an upper plate 12 of a cover body 11 which is formed of a thermoplastic resin such as polypropylene, whose lower side is opened, breaking parts 17 are formed with a hollow part 17a with the section of almost circular form, and thin thickness parts 17b and 17c neighboring the outer side and the inner side of the hollow part 17a. Furthermore, installing pieces 16 in which through holes 15 are formed from the end of the wall 14 of the cover bag 11 are projected. Consequently, when an air bag is expanded, the thin thickness parts 17b and 17c are broken by the expansion pressure, so as to develop the air bag smoothly, and the air bag can be manufactured in a good appearance.