To prescribe a gas leakage position in the case of an internal pressure abnormally rising, adjust the internal pressure by releasing gas partly to the outside when the internal pressure exceeds a fixed value.

Passing a through hole 56 drilled in a rail inner panel 40, a protruded part 16E formed in an upper part of an air bag unit 16 is inserted to the inside of a roof side rail 28. In a protruded direction intermediate part of the protruded part 16E, a squeezed part 60 is formed as the weakest part along the vehicle longitudinal direction to narrow an internal width. Consequently, when an internal pressure of the air bag unit 16 is abnormally increased, stress concentrated in this squeezed part 60 is increased to a prescribed value or more, the squeezed part 60 is bursted. That is, the internal pressure of the air bag unit 16 can be adjusted, and by setting a leakage position of gas to a location isolated from an occupant of the air bag unit 16, leakage gas can be prevented from being blown on the occupant.