The knee-protecting airbag apparatus is mounted on a lower region of a column cover covering a steering column of vehicle projecting rearward from an instrument panel. The airbag (47) folded and housed in a housing emerges and spreads in four directions from the housing for deployment in front of knees of a driver when fed with inflation gas from an inflator (41) that is stored inside the airbag (47) and disposed to the rear side of the housing. Folding of the airbag (47) before being stored in the housing is such that left and right areas of a region (50) attached to the housing is folded on the side of the column side wall (49) to reduce the lateral width L2 of the airbag (47) without superimposing the left and right areas of the mounting region (50) on the inflator (41) in the vertical direction and a folded region (63) on the front side of the mounting region (50) is located in parallel with and in front of the inflator (41).