A knee airbag device includes a knee airbag that is disposed in a folded state inside a steering column cover inside which a projection body that projects downward toward one side in a vehicle-width direction is disposed in a rear area, and a cylindrical inflator that supplies the gas to the knee airbag to deploy the knee airbag. The inflator is disposed inside the steering column cover such that at least a portion of the inflator is positioned in the rear area inside the steering column cover. The inflator is disposed in substantially parallel with an axis of a column tube such that the inflator is positioned below the column tube. The end portion of the inflator, in which a gas injection port is formed, faces toward a rear of the vehicle.