In a knee protecting airbag device (M1), an airbag (26) folded and housed below a steering column (3) is expanded and inflated while rising from the lower side of the steering column (3) along a column cover lower surface (9a) during an inflation process. The shape of the airbag having completed the inflation is in a generally plate shape capable of covering at least the lower surface of the column cover. Even if the knee of the driver comes close to the column cover when the driver depressing a brake pedal, therefore, the airbag being expanded and inflated goes smoothly into the clearance between the knee of the driver and the column cover lower surface. The airbag having completed the expansion and inflation protects the knee of the driver properly so that the knee may not interfere with the column cover.