In the knee protection airbag device of the present invention, an airbag folded and housed in a case opened rearward of the vehicle pushes open the door portion of an airbag cover assembled with the case, upon inflow of inflation gas from the inflator. Around the airbag cover, there is arranged an interior decoration member for vehicle which is provided separate from the airbag cover. The door portion includes at least an upper door openable upward. The upper door is formed in such a size as to cover a gap formed between the interior decoration member and the airbag cover when opened upon deployment of the airbag.