An externally-developed airbag device is provided in which side bag members are expanded along pillars in expanding the airbag and in which even after the airbag has been fully expanded, the frontal view at the front side of the airbag from a driver seat is secured. The airbag can include a lower bag member that is expandable in the width direction (in the left-right direction) of a vehicle along a cowl top portion of the vehicle, left and right side bag members being expandable to be protruded in the upward direction along left and right pillars of the vehicle from both of the left and right ends of the lower bag member, and connecting members provided in an angle portion, in one form, between the respective side bag members and the lower bag member for connecting the side bag members and the lower bag member.