An airbag device includes: a gas generator including: an inflator having one end portion to which a connected part is provided; and a cylindrical retainer configured to cover the inflator; and an airbag including a bag main body having an insertion opening. A part of an outer end part of the retainer inserted in the insertion opening is arranged outside the bag main body together with the connected part, and a remaining part of the retainer is arranged inside the bag main body. The outer end part of the retainer is formed with an extension part extending in a direction of being separated away from the gas blowout part. The bag main body has a clearance filling part configured to fill a part of a clearance between the inflator and the retainer, and the clearance filling part is sandwiched by the inflator and the extension part.