An airbag apparatus receives an occupant's left breast by an expanded left airbag section and receives the right breast by a right airbag section. A void space of the expanded airbag opposes the lateral center of the occupant's breast. Both the left and right airbag sections expand smoothly and substantially evenly. The left and right airbag sections are folded firstly to obtain a first folded body elongated in the fore-and-aft direction and, subsequently, the distal sides from a joint belt, which connect the midsections in the expanding direction of the left and right airbag sections, are opened laterally apart from each other. The rear sides from the joint belt are folded so as to be reduced in width in the fore-and aft direction to obtain the second folded body, and then the lateral width is reduced by the third folding operation to obtain the final folded body.