In an airbag system for a steering wheel according to the invention, an airbag is folded through a first step and a second step after the airbag is unfolded flatly with a driver side wall and a vehicle body side wall superposed each other. Before the first step, the airbag is folded through a preliminarily-folding step to fold back a front edge side region to the vehicle body side wall along a folded line in a transverse direction. In the preliminarily-folded airbag, the folded front edge side is arranged within a range of 20% of the inner diameter of the ring part in each of opposite directions in the front-rear direction from a reference point on the inner peripheral edge of the front part of the ring part.