An airbag device comprising: an airbag that is normally folded and is inflated and deployed in emergency; an inflator for supplying gas to the airbag; a retainer for fixing the airbag and the inflator; and an airbag cover for housing the airbag, wherein the airbag cover includes a plate portion that forms an interior surface of a vehicle and an inner case that is disposed on a back surface of the plate portion and that forms an inflation and deployment path of the airbag, the inner case having first wall surfaces facing each other in a front-rear direction and second wall surfaces facing each other in a right-left direction, wherein the inner case has a plurality of holding holes for holding the retainer, wherein: the retainer has a box shape with an open top and includes a bottom surface portion for supporting the airbag and the inflator, a side wall portion (42; 42a, 42b) that is disposed at an outer periphery of the bottom surface portion and that has a height, and engagement members (43; 45) that are disposed on the side wall portion (42; 42a, 42b) and that are engageable with the holding holes from the inside of the inner case, wherein a substantially entirety of the retainer is housed in the inner case when the engagement members (43; 45) are engaged with the holding holes, characterized in that the height of the side wall portion is set within a range in which the side wall portion does not exceed a line segment connecting a fixed point of the airbag located in the bottom surface portion and a contact point at the first wall surface located between the airbag and the inner case during an inflation and deployment time but before the plate portion is torn open. an airbag that is normally folded and is inflated and deployed in emergency; an inflator for supplying gas to the airbag; a retainer for fixing the airbag and the inflator; and an airbag cover for housing the airbag, wherein the airbag cover includes a plate portion that forms an interior surface of a vehicle and an inner case that is disposed on a back surface of the plate portion and that forms an inflation and deployment path of the airbag, the inner case having first wall surfaces facing each other in a front-rear direction and second wall surfaces facing each other in a right-left direction, a plate portion that forms an interior surface of a vehicle and an inner case that is disposed on a back surface of the plate portion and that forms an inflation and deployment path of the airbag, the inner case having first wall surfaces facing each other in a front-rear direction and second wall surfaces facing each other in a right-left direction, wherein the inner case has a plurality of holding holes for holding the retainer, wherein: the retainer has a box shape with an open top and includes a bottom surface portion for supporting the airbag and the inflator, a side wall portion (42; 42a, 42b) that is disposed at an outer periphery of the bottom surface portion and that has a height, and engagement members (43; 45) that are disposed on the side wall portion (42; 42a, 42b) and that are engageable with the holding holes from the inside of the inner case, wherein a substantially entirety of the retainer is housed in the inner case when the engagement members (43; 45) are engaged with the holding holes, characterized in that the height of the side wall portion is set within a range in which the side wall portion does not exceed a line segment connecting a fixed point of the airbag located in the bottom surface portion and a contact point at the first wall surface located between the airbag and the inner case during an inflation and deployment time but before the plate portion is torn open. a bottom surface portion for supporting the airbag and the inflator, a side wall portion (42; 42a, 42b) that is disposed at an outer periphery of the bottom surface portion and that has a height, and engagement members (43; 45) that are disposed on the side wall portion (42; 42a, 42b) and that are engageable with the holding holes from the inside of the inner case, wherein a substantially entirety of the retainer is housed in the inner case when the engagement members (43; 45) are engaged with the holding holes, characterized in that the height of the side wall portion is set within a range in which the side wall portion does not exceed a line segment connecting a fixed point of the airbag located in the bottom surface portion and a contact point at the first wall surface located between the airbag and the inner case during an inflation and deployment time but before the plate portion is torn open.