Airbag apparatus and method of folding an airbag

In order to provide an airbag apparatus wherein both a left airbag section (12) and a right airbag section (14) expand smoothly and substantially evenly from the initial stage of expansion,     the left airbag section (12) and the right airbag section (14) 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 (18), which connects the midsections in the expanding direction of the left airbag section (12) and the right airbag section (14), are opened laterally apart from each other and the rear sides from the joint belt (18) 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.