An airbag apparatus includes an airbag that has a first portion and a second portion. The airbag is folded such that the first portion approaches the second portion. The folded airbag has an elongated shape extending along a first direction. An inlet 12a is formed in the airbag 5. By receiving the supply of gas through the inlet 12a, the folded airbag 5 is inflated while being deployed in a second direction perpendicular to the first direction, such that the first portion moves away from the second portion. The inlet 12a is arranged in a part of the folded airbag 5 that is located in a trailing side in the second direction. A first attachment portion 16 is provided in the same part of the folded airbag 5. A second attachment portion 27 is provided in each of the ends of the folded airbag 5. The folded airbag 5 is installed in a ceiling of a vehicle 2 by means of the first attachment portion 16 and the second attachment portions 27. When the airbag 5 is being deployed and inflated, the second attachment portions 27 function to apply an outward tension along the first direction to the airbag 5.