The airbag apparatus includes a case for housing and holding an airbag and an inflator. The case includes a vent opening for releasing inflation gas filled inside the airbag and a lid operated by an actuator for closing and opening the vent opening. The lid is pivotally supported at a periphery of the vent opening for rotation about its rotary shaft disposed across the vent opening, and includes an outward-opening area adapted to rotate outward of the case and an inward-opening area adapted to rotate inward of the case, with a border of the outward-opening area and inward-opening area disposed on the rotary shaft. A sliding element is connected to the actuator for holding the lid to keep the vent opening closed when the actuator is inactive and for sliding movement to release the lid and open the vent opening upon operation of the actuator.