A knee airbag device includes a knee airbag that is disposed in a folded state inside a steering column cover. When the knee airbag receives gas, the knee airbag is inflated and deployed from an inside of the steering column cover toward knees of an occupant. The thickness of outer portions of the knee airbag, which are positioned outside centers of the knees when the knee airbag is deployed, is set to be larger than the thickness of a general portion of the knee airbag, which is positioned in front of the knees when the knee airbag is deployed.