A knee protection airbag apparatus (S), which accommodates an airbag (73) and an inflator (62) and is opened at a vehicle rear side part thereof and has a casing assembly (22) fixedly attached to a vehicle body (B) and also has an airbag cover (46) that has a peripheral portion (47), whose upper part is assembled to a vehicle interior member (19b), and that is assembled to the casing assembly (22). The knee protection airbag apparatus (S) is mounted on a vehicle by assembling the airbag (73), the inflator (62), and the airbag cover (46) to the casing assembly (22). The airbag cover (46) is assembled to a knee panel (27) of the casing assembly (22) in such a manner as to be able to move only in a nearly upward or downward direction and in a nearly left-right direction so as to be enabled to absorb an error in assembling the airbag cover (46) to the interior member to be placed thereabove.