An externally developed airbag device is provided that reliably deploys along predetermined areas. More particularly, an airbag includes a bag base, which deploys along the lower edge of the windshield, and bag side segments, which deploy while extending upwardly from the right and left sides of the bag base along the exteriors of "A" pillars. Guide rods vertically extend along the exteriors of the "A" pillars. Each of the bag side segments has a connecting strap on its top that connects the bag side segment to the guide rod. The connecting straps are movable along the guide rods. The bag side segments are guided along the guide rods by the connecting straps and deploy along the exteriors of the "A" pillars.