Abstract: An airbag apparatus of a vehicle is provided with an airbag deployed and inflated mainly in a portion of a passenger compartment of the vehicle rearward of a rear seat. Prior to deployment and inflation, the airbag has a bellows portion formed by folding a portion of the airbag near a gas inlet port into an accordion shape, and a roll portion formed by folding a portion of the airbag far from the gas inlet port into a roll shape, and is accommodated between a roof panel and a roof headlining of a vehicle in such a manner that the gas inlet port is positioned on a front side. Between the roof panel and the roof headlining, the bellows portion is arranged substantially above or substantially below the roll portion in such a manner that the flat sections are overlapped with each other substantially in a vertical direction.