A knee protection airbag device includes an airbag, an inflator, an airbag cover, and an attachment base which holds those and is to be attached to a vehicle body member. The attachment base includes a holding seat which is disposed in front of a body portion of the inflator and a folded portion of the airbag, and holds the body portion and the airbag attached by a fastening bolt of the inflator, and attachment seats which are arranged on left and right sides of the holding seat and are attached to the vehicle body member. The airbag cover covers a vicinity of the airbag folded and the body portion of the inflator on a rear side of the attachment base, is attached to the attachment base due to the fastening bolt being fastened, and provides a projection opening on a bottom side of the airbag cover.