A knee airbag device including an airbag cushion, a storage case in which is formed a horizontally long opening part where the expanded and deployed airbag cushion pops outs, a first flap for covering the airbag cushion exposed through the opening part such that the airbag cushion is retained in the storage case, and having a first weak part that breaks when pressed by the airbag cushion during expansion and deployment, and a second flap, which is a component with a larger dimension in a length direction than the first flap and for covering an outer surface of the first flap so as to transverse in a short direction of the opening part a center part of the airbag cushion in a lateral direction, the second flap having a second weak part that breaks when pressed by the airbag cushion after rupture of the first weak part.