An airbag of the knee-protecting airbag device according to the present invention includes a cover area which covers the lower side or the back side of key accessories and is contactable with the accessories. The airbag includes a bag-shaped airbag body with a driver's side wall and a column side wall, a protecting cloth, and a retainer cloth. The protecting cloth is joined at its lower end to an area in the column side wall below the cover area, while leaving its upper end as free end. The retainer cloth is so joined at its lower end to the driver's side wall as to be capable of covering the upper end part of the protecting cloth as extended to the driver's side wall. The airbag is transversely folded and housed in the housing in a condition that the upper end part of the protecting cloth is extended to the driver's side wall and then the retainer cloth is superposed thereon.