In a knee airbag attached to a steering column, the knee airbag is inflated and deployed quickly to a small space between an instrument panel and the knees of a vehicle occupant in a collision.A knee airbag 36 of a knee airbag device 10 is formed of a first inflation portion 38 which extends in a vehicle lateral direction along an instrument panel 12 side, and a second inflation portion 46, which is deployed to a vehicle occupant knee side. A gas supply opening 42, to which gas ejected from an inflator 22 is supplied, is disposed at a first inflation portion 38 side. The gas supply opening 42, and the pair of protection portions 46 provided at left and right end portions of knee airbag 36, are provided on substantially the same line in a vehicle lateral direction, and the first inflation portion 36 communicates the two as a gas flow path. Therefore, in a frontal collision, even if a space 48 between the instrument panel 12 and the knees of the vehicle occupant is small, the pair of protection portions 46 can be quickly inflated and deployed to the small space 48.