An airbag device includes a tether having an improved folding position that enables the tether to unfold as it cuts through a covering. The airbag device may include a foam pad embedded in a seatback, an airbag cushion folded at a lateral side of the foam pad and configured to be deployed forward, a covering shaped to cover the foam pad and configured to cover the seatback, and a tether positioned between the foam pad and the covering and connected between the seatback and the airbag cushion. The tether is configured to unfold as it cuts through the covering when the airbag cushion is deployed.