A knee airbag device includes: a knee airbag that is housed in a folded state at a lower end portion of an instrument panel, and that, due to gas being supplied thereto, is inflated and expanded along the instrument panel; an airbag case that houses the knee airbag; an airbag door portion that structures a lower surface of the airbag case, and that, due to a planned rupture portion rupturing due to inflation pressure of the knee airbag, is opened toward a vehicle front side; and a flap that is housed within the airbag case, one end portion of the flap being fixed to the airbag case, another end portion of the flap being a free end, and that, at a time of inflation and expansion of the knee airbag, unfolds, and covers the airbag door portion from a vehicle rear side.