The airbag apparatus includes an airbag, a tether, a vent mechanism and an actuator. When the actuator is activated, a portion of the tether is retained by a movable member of the actuator to reduce the degree of protrusion of the airbag while the vent mechanism is released from the movable member to open up a vent opening and increase emission of inflation gas. When the actuator is inactive, the portion of the tether is detached from a position where the portion would otherwise be retained by the movable member to increase the degree of protrusion of the airbag while the vent mechanism is retained by the movable member to close off the vent opening and reduce the emission of gas. Further, the actuator may be activated at a delayed timing to open up the vent opening to increase the gas emission after the tether is configured to increase the protruding degree of the airbag.