To instantaneously extend an air bag door part without increasing an inflating pressure of an air bag body, even in the case of molding the air bag door part of interior trimming member for a vehicle by hard resin.

In an air bag case 14, an end part 30A of a steel plate 30 is mounted, and in the other end part 30C of the steel plate 30, a breaking edge tool 36 is fixed. An edge part 36B of the breaking edge tool 36 is in proximity to a bottom part 24A of a groove 24, an intermediate part of the breaking edge tool 36 is formed in a spherical bending support point part 36C. Consequently, by thrust shock force of an air bag body 18, the edge part 36B of the breaking edge tool 36 forms a notch in the bottom part 24A of the groove 24, also by the bending support point part 36C of the breaking edge tool 36, force in mutually separating direction acts in a bottom part side end part of both side wall parts of the groove 24, and an air bag door part 20 is broken along the groove 24, to be extended.