An airbag of a knee-protecting airbag device is mounted on a case together with an inflator with the aid of mounting means of the inflator. The case is provided with an airbag emergence opening at the lower end region. The airbag includes a vehicle body side panel, an occupant side panel, a mounting inflatable region which is located in a lower end region of the airbag as deployed and is secured to the case, and a knee-protecting region deployable in such a manner as to extend rearwardly and upwardly from the mounting inflatable region. The airbag further includes inside the mounting inflatable region a tether which joins the vehicle body side panel and occupant side panel for regulating a clearance between the vehicle body side panel and occupant side panel at airbag deployment.