To provide an externally expansion type air bag device smoothly expanding an air bag without hooking it on a corner edge such as an opening for swelling the air bag.

This externally expansion type air bag device 1 comprises a vessel-like retainer 2 having an opening in its upper surface, the air bag 4 folded and disposed in the retainer 2, a gas generator 6 for air bag expansion, and a seat 8 mounted to the edge of the upper surface opening of the retainer 2 so as to cover the air bag 4. The air bag 4 swells out of the car body through the opening 11 for swelling the air bag of a hood 10, and expands and develops along the outer surface of the hood 10. In a normal time, the seat 8 covers a folded body of the air bag 4. When the air bag 4 expands, the seat 8 extends following the air bag 4 and covers the corner edge of the opening 11 for swelling the air bag.