A knee airbag device mounted on a steering column has an inner case in which a folded knee airbag is housed, and an outer case that is placed over the inner case. The outer case is fixed to a lower side of a vehicle-rear-side portion of a steering column that moves relative to a vehicle-front-side portion when a load equal to or greater than a threshold load is applied to the steering column in a direction of the axis of the steering column, and has a rigidity with which the shape of the folded knee airbag is maintained under normal conditions. The outer case has an opening formed in a region where the outer case would interfere with the vehicle-front-side portion when the vehicle-rear-side portion moves relative to the vehicle-front-side portion.