A knee protecting airbag device in which a folded airbag and an inflator are housed and held in a case. In the inflator, a connection portion to be connected with a connector having leadwires connected is exposed to the outside of the case through insert holes formed in the airbag and in the case. The through hole of the case has an internal diameter size smaller than the external diameter size of the general portion of the inflator near the connection portion of the inflator.