Airbag apparatus includes an airbag having at least two bag body sections superposed on each other. Gas flow-in portion for connection to an inflator is formed in one of the bag body sections adjacent to one edge of the one bag body section. Communicating hole is formed in the plurality of bag body sections adjacent to the other edge of the one bag body section. Upon activation of the inflator, gas is supplied into the one bag body section, through the gas flow-in portion, to develop the one bag body section. After that, the other bag body section superposed on the one bag body section is developed by the gas supplied through the communicating hole.