A knee protection airbag device includes: an airbag that is folded and stored; an inflator configured to supply inflation gas to the airbag; a case that stores the airbag and the inflator; and an airbag cover covering the folded airbag. The airbag cover includes a door portion that covers a protruding opening and can be opened when the airbag is deployed and inflated, and is attached to a peripheral wall portion of the case around the door portion. A recessed groove through which a harness main body can be inserted is formed at a portion of a peripheral edge of the protruding opening in the case so as to recess an end edge of the peripheral wall portion. A bead for reinforcement at a peripheral edge portion of the recessed groove is formed on the peripheral wall portion so as to surround a periphery of the recessed groove.