In an airbag device (10) wherein within a casing (12) opening upwardly an inflator (16) and an airbag (14) are housed at its bottom part and the opening side, respectively, a cylindrical diffuser (18) surrounding the outer circumference of the inflator (16) is provided, which has a partition wall (40) projecting toward the opening side of the casing and partitioning an airbag-housing space into a forward side and a rearward side relative to the vehicle, in which to house the airbag (14) dividedly, and a lip portion (32) of the airbag (14) is superposed downwards of the diffuser (18) thereby to lock the bottom part of the diffuser (18) to the bottom surface of the casing (12). Thus, it is easy, albeit compart, to control the deployment behavior of the airbag and excellent in workability of assembling the airbag in the casing.