An airbag apparatus may include an airbag module mounted at a lower end portion of a crash pad; an airbag cushion configured to deploy from the airbag module to deploy in a space between a knee portion and a head portion of a passenger, wherein a tether is connected to the airbag cushion in a deployment direction of the airbag cushion toward at least one of the knee portion, a chest portion, and the head portion of the passenger to form an integrated deployment shape of the airbag cushion; and a release device connected to the tether close to the airbag module, and operated to allow the tether to be released or maintained in a fixed state in a deployment process of the airbag cushion depending on a body shape of the passenger on a seat of the vehicle.