An airbag for a knee airbag apparatus which is rapidly deployed upward along a shape of an instrument panel includes a front panel; and a rear panel coupled to the front panel to form an inflation chamber. The front panel neighbors to the instrument panel and the rear panel neighbors to knees of a passenger when the airbag is inflated. The rear panel includes a wrinkle portion which is deployed at a position corresponding to a housing connecting portion to which the instrument panel and a rear end of the housing neighbor when the airbag is inflated, so that an additional inflation chamber is formed.