PURPOSE: To provide a small-sized air bag device free from the necessity of increasing the thickness of the air bag as a whole, as for the air bag device installed on the car body side part.

CONSTITUTION: As for an air bag device 27 having an air bag 31 which is installed on a roof side rail 11 and expands and is developed inside the window part of a car body side part by detecting the impact force in the car width direction, a seat (penetration resistive member) 41 for increasing the penetration resistance of the side surface on the window part 17 side of the air bag 31 is installed on the side surface. 31 and a window part 17 and can be shifted to the crossing state with the window part, and a shifting means 37 for shifting the supporting member 35 by detecting the impact force are provided.