In the knee protection air bag apparatus, the air bag cover is assembled to a case which stores therein an air bag and an inflator and also includes an opening formed in the vehicle backward direction portion thereof such that the air bag cover is movable with respect to the case in the vehicle back-and-forth direction, in the vehicle vertical direction and in the vehicle right and left direction and, at the same time, the air bag cover is mounted on its adjoining vehicle interior part (a lower panel). A projecting streak provided on the air bag cover regulates the movement of the air bag cover in the vehicle backward direction with respect to the case, whereby the expanding and developing power of the air bag can be properly transmitted to a breaking portion formed in the lid portion of the air bag cover.