The knee-protecting airbag apparatus includes an airbag module and a mounting bracket for fixing the airbag module on a vehicle body. The mounting bracket is provided with a mounting hole that is adapted to be fastened to a mounting seat of the vehicle body. The airbag module includes a retaining lug for fixing the airbag module tentatively on the vehicle body. When the retaining lug is engaged with a periphery of a retaining hole of the vehicle body, a rearward rotation moment is generated to rotate the airbag module about the engaging point of the retaining lug and periphery of the retaining hole, so that a pressing plane of the airbag module formed above the retaining lug abuts on and gets supported by a receiving plane formed on the vehicle body such that the mounting hole of the mounting bracket is positioned on a fastening location of the mounting seat of the vehicle body.