An internal airbag device may include an inflator, an airbag housing mounted in a roof in a region between a front seat and a rear seat, and an airbag received in the airbag housing to deploy downwards by gas supplied from the inflator, wherein the airbag includes a horizontal airbag section supported by the roof during the gas may be supplied therein in a deployment thereof, and a vertical airbag section fluid-connected to the horizontal airbag section and extending downwards from an end of the horizontal airbag section opposite the rear seat in a deployment thereof.