An airbag device includes an airbag, an adjusting unit capable of adjusting an aperture efficiency of at least one vent hole, a tether belt whose one end is connected to the adjusting unit, and a holding unit for hooking another end of the tether belt. In case where the inflating airbag needs to exert a small reactive force, the adjusting unit is made in a full-opened state at a development early stage with the other end being held by the holding unit and then made in a half-opened until a development final stage with the other end being kept held by the holding unit. In case where the inflating airbag needs to exert a large reactive force, the adjusting unit is made in a full-closed state from a development start to the development final stage with the other end being released from the holding unit.