An airbag device where a case and a lid are connected by engaging a hook of the case with a hook hole in the lid and where the hook moves in the hook hole toward the vehicle interior side when the lid recedes. Energy absorption characteristics when the lid recedes are improved by a simple structure. The hook (18) used for connection to the lid (20) and provided at the case (10) is engaged with the hook hole (25) of the lid (20). The hook hole (25) is a long hole elongated in the vertical direction, and in the middle of the long hole is provided a partition-like section (26) for partitioning the inside of the hook hole (25) into the upper half and the lower half. The hook (18) is engaged with that portion of the hook hole (25) which is lower than the partition-like section (26). As the lid (20) recedes, the hook (18) moves upward in the hook hole (25) while cutting the partition-like section (26).